btc: support timestamp locktimes

### CHANGELOG.md
@@ -10,6 +10,7 @@ customers cannot upgrade their bootloader, its changes are recorded separately.
 - Add support for BitBoxSync
 - Display long transaction and swap amounts in full instead of truncating them
 - Bitcoin: allow signing messages with keys in the m/45' application namespace
+- Bitcoin: support timestamp-based absolute locktimes and remove the RBF status display
 
 ### v9.26.5
 - Security improvements

### messages/btc.proto
@@ -103,7 +103,8 @@ message BTCSignInitRequest {
   uint32 version = 4; // must be 1 or 2
   uint32 num_inputs = 5;
   uint32 num_outputs = 6;
-  uint32 locktime = 7; // must be <500000000
+  // Consensus nLockTime: values below 500000000 are block heights, otherwise Unix timestamps.
+  uint32 locktime = 7;
 
   enum FormatUnit {
     // According to `coin` (BTC, LTC, etc.).

### py/bitbox02/bitbox02/bitbox02/bitbox02.py
@@ -458,17 +458,21 @@ def btc_sign(
           if `btc_sign_needs_prevtxs()` returns True.
         outputs: transaction outputs. Can be an external output
         (BTCOutputExternal) or an internal output for change (BTCOutputInternal).
-        version, locktime: reserved for future use.
+        version: transaction version, 1 or 2.
+        locktime: consensus nLockTime, interpreted as a block height or Unix timestamp.
         format_unit: defines in which unit amounts will be displayed
         output_script_configs: script types for outputs belonging to the same keystore
         Returns: list of (input index, signature) tuples.
         Raises Bitbox02Exception with ERR_USER_ABORT on user abort.
         """
         # pylint: disable=no-member,too-many-branches,too-many-statements
 
-        # Reserved for future use.
         assert version in (1, 2)
 
+        if locktime >= 500_000_000:
+            # Timestamp-based nLockTime supported since v9.27.0.
+            self._require_atleast(semver.VersionInfo(9, 27, 0))
+
         if any(map(is_taproot, script_configs)):
             self._require_atleast(semver.VersionInfo(9, 10, 0))
 

### py/bitbox02/bitbox02/communication/generated/btc_pb2.pyi
@@ -345,7 +345,7 @@ class BTCSignInitRequest(google.protobuf.message.Message):
     num_inputs: builtins.int
     num_outputs: builtins.int
     locktime: builtins.int
-    """must be <500000000"""
+    """Consensus nLockTime: values below 500000000 are block heights, otherwise Unix timestamps."""
     format_unit: global___BTCSignInitRequest.FormatUnit.ValueType
     contains_silent_payment_outputs: builtins.bool
     @property

### src/rust/bitbox-proto/src/generated/shiftcrypto.bitbox02.rs
@@ -670,7 +670,7 @@ pub struct BtcSignInitRequest {
     pub num_inputs: u32,
     #[prost(uint32, tag = "6")]
     pub num_outputs: u32,
-    /// must be <500000000
+    /// Consensus nLockTime: values below 500000000 are block heights, otherwise Unix timestamps.
     #[prost(uint32, tag = "7")]
     pub locktime: u32,
     #[prost(enumeration = "btc_sign_init_request::FormatUnit", tag = "8")]

### src/rust/bitbox-test-vectors/src/btc_transaction/cases/additional_psbt.rs
@@ -561,7 +561,7 @@ fn high_address_index() -> TestVector {
     )
 }
 
-fn locktime(block: u32, sequence: u32, rbf: bool) -> TestVector {
+fn locktime(block: u32, sequence: u32, previous_body: &str) -> TestVector {
     let spend = simple_spend(
         SimpleType::P2wpkh,
         "m/84'/1'/0'",
@@ -570,23 +570,24 @@ fn locktime(block: u32, sequence: u32, rbf: bool) -> TestVector {
         block,
         p2tr_script(TBTC_EXTERNAL_XONLY),
     );
-    let qualifier = if rbf { "rbf" } else { "non-rbf" };
+    let previous_locktime_screen = Screen::Confirm {
+        title: String::new(),
+        body: previous_body.into(),
+        longtouch: false,
+    };
     let locktime_screen = Screen::Confirm {
         title: String::new(),
-        body: format!(
-            "Locktime on block:\n{block}\nTransaction is {}RBF",
-            if rbf { "" } else { "not " }
-        ),
+        body: format!("Locktime on block:\n{block}"),
         longtouch: false,
     };
     transaction_vector(
-        &format!("locktime-{qualifier}"),
-        &format!("Displays block locktime {block} and its {qualifier} sequence semantics."),
+        &format!("locktime-sequence-{sequence:08x}"),
+        &format!("Displays block locktime {block} for transaction sequence {sequence:#010x}."),
         Coin::Tbtc,
         spend.psbt,
         PsbtSignOptions::default(),
         vec![spend.expected_signature],
-        screens::simple_tbtc(&[locktime_screen]),
+        screens::simple_tbtc_locktime(&[previous_locktime_screen], &[locktime_screen]),
     )
 }
 
@@ -601,7 +602,7 @@ fn zero_locktime() -> TestVector {
     );
     transaction_vector(
         "locktime-zero",
-        "Suppresses the locktime confirmation when locktime is zero even if the sequence signals RBF.",
+        "Suppresses the locktime confirmation when locktime is zero even if the sequence is non-final.",
         Coin::Tbtc,
         spend.psbt,
         PsbtSignOptions::default(),
@@ -670,7 +671,7 @@ fn silent_payment_rejects_owned_output() -> TestVector {
     )
 }
 
-fn ltc_locktime(sequence: u32, qualifier: &str) -> TestVector {
+fn ltc_locktime(sequence: u32) -> TestVector {
     let spend = simple_spend(
         SimpleType::P2wpkh,
         "m/84'/2'/0'",
@@ -679,19 +680,24 @@ fn ltc_locktime(sequence: u32, qualifier: &str) -> TestVector {
         10,
         ltc_external_script(),
     );
-    let locktime_screen = Screen::Confirm {
+    let previous_locktime_screen = Screen::Confirm {
         title: String::new(),
         body: "Locktime on block:\n10\n".into(),
         longtouch: false,
     };
+    let locktime_screen = Screen::Confirm {
+        title: String::new(),
+        body: "Locktime on block:\n10".into(),
+        longtouch: false,
+    };
     transaction_vector(
-        &format!("locktime-litecoin-{qualifier}"),
-        "Displays a Litecoin block locktime without Bitcoin-specific RBF wording.",
+        &format!("locktime-litecoin-sequence-{sequence:08x}"),
+        &format!("Displays a Litecoin block locktime for transaction sequence {sequence:#010x}."),
         Coin::Ltc,
         spend.psbt,
         PsbtSignOptions::default(),
         vec![spend.expected_signature],
-        screens::simple_ltc(&[locktime_screen]),
+        screens::simple_ltc_locktime(&[previous_locktime_screen], &[locktime_screen]),
     )
 }
 
@@ -706,10 +712,18 @@ pub fn all() -> Vec<TestVector> {
         p2wpkh_p2sh(),
         high_address_index(),
         zero_locktime(),
-        locktime(10, Sequence::MAX.0 - 1, false),
-        locktime(10, Sequence::MAX.0 - 2, true),
-        ltc_locktime(Sequence::MAX.0 - 1, "non-rbf-sequence"),
-        ltc_locktime(Sequence::MAX.0 - 2, "rbf-sequence"),
+        locktime(
+            10,
+            Sequence::MAX.0 - 1,
+            "Locktime on block:\n10\nTransaction is not RBF",
+        ),
+        locktime(
+            10,
+            Sequence::MAX.0 - 2,
+            "Locktime on block:\n10\nTransaction is RBF",
+        ),
+        ltc_locktime(Sequence::MAX.0 - 1),
+        ltc_locktime(Sequence::MAX.0 - 2),
         p2tr_output_btc(),
         silent_payment_rejects_owned_output(),
         op_return_nonascii(),

### src/rust/bitbox-test-vectors/src/btc_transaction/cases/descriptor_psbt.rs
@@ -8,7 +8,7 @@ use super::common::{
 };
 use super::screens;
 use crate::btc_transaction::{
-    Coin, KeyOriginInfo, MultisigScriptType, PsbtSignOptions, Registration, ScriptConfig,
+    Coin, KeyOriginInfo, MultisigScriptType, PsbtSignOptions, Registration, Screen, ScriptConfig,
     ScriptConfigWithKeypath, TestVector,
 };
 use bitcoin::bip32::{ChainCode, ChildNumber, DerivationPath, Fingerprint, KeySource, Xpriv, Xpub};
@@ -243,6 +243,8 @@ fn policy_spend(policy: &str, change_index: u32, sign_cosigner: bool) -> PolicyS
     let our_xpub = simulator_xpub_at(&secp, &account);
     let cosigner_root = seeded_xpriv(100);
     let cosigner_xpub = account_xpub(&cosigner_root, &account);
+    let has_cosigner = policy.contains("@1/");
+    assert!(!sign_cosigner || has_cosigner);
     let descriptor = policy
         .replace("/**", "/<0;1>/*")
         .replace(
@@ -265,20 +267,23 @@ fn policy_spend(policy: &str, change_index: u32, sign_cosigner: bool) -> PolicyS
     if sign_cosigner {
         add_cosigner_signature(&mut psbt, &cosigner_root);
     }
+    let mut keys = vec![KeyOriginInfo {
+        root_fingerprint: Some(simulator_xprv().fingerprint(&secp).to_string()),
+        keypath: Some(keypath(&account)),
+        xpub: our_xpub.to_string(),
+    }];
+    let mut display_keys = vec![format!("This device: {}", screens::DEVICE_POLICY_XPUB)];
+    if has_cosigner {
+        keys.push(KeyOriginInfo {
+            root_fingerprint: None,
+            keypath: None,
+            xpub: cosigner_xpub.to_string(),
+        });
+        display_keys.push(cosigner_xpub.to_string());
+    }
     let config = ScriptConfig::Policy {
         policy: policy.into(),
-        keys: vec![
-            KeyOriginInfo {
-                root_fingerprint: Some(simulator_xprv().fingerprint(&secp).to_string()),
-                keypath: Some(keypath(&account)),
-                xpub: our_xpub.to_string(),
-            },
-            KeyOriginInfo {
-                root_fingerprint: None,
-                keypath: None,
-                xpub: cosigner_xpub.to_string(),
-            },
-        ],
+        keys,
     };
     let input_branch = if policy.contains("<10;11>") { 10 } else { 0 };
     let input_path = path(&format!("{account}/{input_branch}/0"));
@@ -287,10 +292,7 @@ fn policy_spend(policy: &str, change_index: u32, sign_cosigner: bool) -> PolicyS
         config,
         account,
         input_pubkey: simulator_xpub_at(&secp, &input_path).public_key,
-        display_keys: vec![
-            format!("This device: {}", screens::DEVICE_POLICY_XPUB),
-            cosigner_xpub.to_string(),
-        ],
+        display_keys,
     }
 }
 
@@ -570,6 +572,42 @@ fn policy_tr_unspendable_internal_key_complex() -> TestVector {
     })
 }
 
+fn policy_timestamp_locktime() -> TestVector {
+    const LOCKTIME: u32 = 1_800_000_000;
+
+    let policy = "wsh(and_v(v:pk(@0/**),after(1800000000)))";
+    let name = "test timestamp locktime";
+    let mut spend = policy_spend(policy, 0, false);
+    spend.psbt.unsigned_tx.lock_time = bitcoin::absolute::LockTime::from_consensus(LOCKTIME);
+    spend.psbt.unsigned_tx.input[0].sequence = Sequence(Sequence::MAX.0 - 1);
+    let locktime_screen = Screen::Confirm {
+        title: String::new(),
+        body: "Locktime (UTC):\n2027-01-15\n08:00:00".into(),
+        longtouch: false,
+    };
+    let mut vector = transaction_vector(
+        "policy-timestamp-locktime",
+        "Signs and finalizes a registered WSH policy whose absolute locktime is a Unix timestamp.",
+        Coin::Tbtc,
+        spend.psbt,
+        PsbtSignOptions {
+            force_script_config: Some(ScriptConfigWithKeypath {
+                script_config: spend.config.clone(),
+                keypath: keypath(&spend.account),
+            }),
+            ..Default::default()
+        },
+        vec![ecdsa_signature(0, spend.input_pubkey)],
+        screens::policy_timestamp_locktime(policy, name, &spend.display_keys, locktime_screen),
+    );
+    vector.registrations.push(Registration {
+        script_config: spend.config,
+        keypath: None,
+        name: name.into(),
+    });
+    vector
+}
+
 pub fn all() -> Vec<TestVector> {
     let multipath_policy = "wsh(multi(2,@0/<10;11>/*,@1/<20;21>/*))";
     let standard_policy = "wsh(multi(2,@0/**,@1/**))";
@@ -604,6 +642,7 @@ pub fn all() -> Vec<TestVector> {
         policy_tr_keyspend_with_script_tree(),
         policy_tr_unspendable_internal_key(),
         policy_tr_unspendable_internal_key_complex(),
+        policy_timestamp_locktime(),
         policy_vector(
             "policy-different-multipath-derivations",
             "Signs and finalizes a registered WSH policy whose two keys use different receive and change branches.",

### src/rust/bitbox-test-vectors/src/btc_transaction/cases/screens.rs
@@ -24,6 +24,7 @@ const PSBT_OTHER_ACCOUNT_ADDRESS_GROUPED: &str =
 // only earlier difference would be such an incomplete capture start at v9.20, so their screen lists
 // cannot be mistaken for the screens shown by the device.
 const COMPLETE_TRANSACTION_SCREEN_CAPTURE_VERSION: &str = "9.20.0";
+const LOCKTIME_CONFIRMATION_UPDATE_VERSION: &str = "9.27.0";
 pub const DEVICE_POLICY_XPUB: &str = "[4c00739d/48'/1'/0'/3']tpubDF5MSzQdK2GfjmkNvrCZzpJhFt3if1HmrAdimugmGqWDCXYpkjxHpFZYuDxYYDAnnFMLMjLkMGvij2XV8pLtHBejgGy5RvNW4875nFGBDWv";
 
 fn success(
@@ -195,6 +196,42 @@ fn simple_expectations(
     ]
 }
 
+fn simple_locktime_expectations(
+    previous_middle: &[Screen],
+    current_middle: &[Screen],
+    external_address: &str,
+    grouped_external_address: &str,
+    unit: &str,
+) -> Vec<VersionExpectation> {
+    let amount = format!("0.20000000 {unit}");
+    let total = format!("0.30000000 {unit}");
+    let fee_amount = format!("0.10000000 {unit}");
+    let suffix = |address_value: &str, middle: &[Screen]| {
+        let mut screens = vec![address(&amount, address_value)];
+        screens.extend_from_slice(middle);
+        screens.extend([warning_fee(&total, &fee_amount), high_fee(50), status()]);
+        screens
+    };
+
+    vec![
+        success(
+            Some(COMPLETE_TRANSACTION_SCREEN_CAPTURE_VERSION),
+            Some("9.26.0"),
+            suffix(external_address, previous_middle),
+        ),
+        success(
+            Some("9.26.0"),
+            Some(LOCKTIME_CONFIRMATION_UPDATE_VERSION),
+            suffix(grouped_external_address, previous_middle),
+        ),
+        success(
+            Some(LOCKTIME_CONFIRMATION_UPDATE_VERSION),
+            None,
+            suffix(grouped_external_address, current_middle),
+        ),
+    ]
+}
+
 fn taproot_policy_expectations(prefix: Vec<Screen>) -> Vec<VersionExpectation> {
     let suffix_920 = vec![
         address("0.20000000 TBTC", TBTC_EXTERNAL_ADDRESS),
@@ -237,9 +274,26 @@ pub fn simple_tbtc(middle: &[Screen]) -> Vec<VersionExpectation> {
     )
 }
 
-pub fn simple_ltc(middle: &[Screen]) -> Vec<VersionExpectation> {
-    simple_expectations(
-        middle,
+pub fn simple_tbtc_locktime(
+    previous_middle: &[Screen],
+    current_middle: &[Screen],
+) -> Vec<VersionExpectation> {
+    simple_locktime_expectations(
+        previous_middle,
+        current_middle,
+        TBTC_EXTERNAL_ADDRESS,
+        TBTC_EXTERNAL_ADDRESS_GROUPED,
+        "TBTC",
+    )
+}
+
+pub fn simple_ltc_locktime(
+    previous_middle: &[Screen],
+    current_middle: &[Screen],
+) -> Vec<VersionExpectation> {
+    simple_locktime_expectations(
+        previous_middle,
+        current_middle,
         LTC_EXTERNAL_ADDRESS,
         LTC_EXTERNAL_ADDRESS_GROUPED,
         "LTC",
@@ -566,6 +620,26 @@ pub fn policy(policy: &str, name: &str, keys: &[String], taproot: bool) -> Vec<V
     }
 }
 
+pub fn policy_timestamp_locktime(
+    policy: &str,
+    name: &str,
+    keys: &[String],
+    locktime_screen: Screen,
+) -> Vec<VersionExpectation> {
+    let mut screens = policy_prefix(policy, name, keys);
+    screens.extend([
+        address("0.20000000 TBTC", TBTC_EXTERNAL_ADDRESS_GROUPED),
+        locktime_screen,
+        warning_fee("0.30000000 TBTC", "0.10000000 TBTC"),
+        high_fee(50),
+        status(),
+    ]);
+    vec![
+        invalid_input_before(LOCKTIME_CONFIRMATION_UPDATE_VERSION),
+        success(Some(LOCKTIME_CONFIRMATION_UPDATE_VERSION), None, screens),
+    ]
+}
+
 pub fn policy_wsh() -> Vec<VersionExpectation> {
     policy(
         "wsh(or_b(pk(@0/<0;1>/*),s:pk(@1/<0;1>/*)))",

### src/rust/bitbox-test-vectors/testdata/btc-transaction-test-vectors.json
@@ -2227,7 +2227,7 @@
     },
     {
       "id": "locktime-zero",
-      "description": "Suppresses the locktime confirmation when locktime is zero even if the sequence signals RBF.",
+      "description": "Suppresses the locktime confirmation when locktime is zero even if the sequence is non-final.",
       "coin": "tbtc",
       "psbt": {
         "transaction": "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"
@@ -2302,8 +2302,8 @@
       ]
     },
     {
-      "id": "locktime-non-rbf",
-      "description": "Displays block locktime 10 and its non-rbf sequence semantics.",
+      "id": "locktime-sequence-fffffffe",
+      "description": "Displays block locktime 10 for transaction sequence 0xfffffffe.",
       "coin": "tbtc",
       "psbt": {
         "transaction": "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"
@@ -2347,6 +2347,7 @@
         },
         {
           "min_version": "9.26.0",
+          "max_version_exclusive": "9.27.0",
           "outcome": "success",
           "screens": [
             {
@@ -2378,6 +2379,40 @@
               "body": "confirmed"
             }
           ]
+        },
+        {
+          "min_version": "9.27.0",
+          "outcome": "success",
+          "screens": [
+            {
+              "type": "transaction_address",
+              "amount": "0.20000000 TBTC",
+              "address": "tb1p ff8v kq80 pu2c gtu7 ttga d2zn w62v 2lgu hw6p trpp wns6 nrpq au2q cuz3 7d"
+            },
+            {
+              "type": "confirm",
+              "title": "",
+              "body": "Locktime on block:\n10",
+              "longtouch": false
+            },
+            {
+              "type": "transaction_fee",
+              "amount": "0.30000000 TBTC",
+              "fee": "0.10000000 TBTC",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "High fee",
+              "body": "The fee is 50.0%\nthe send amount.\nProceed?",
+              "longtouch": true
+            },
+            {
+              "type": "status",
+              "title": "Transaction",
+              "body": "confirmed"
+            }
+          ]
         }
       ],
       "expected_signatures": [
@@ -2390,8 +2425,8 @@
       ]
     },
     {
-      "id": "locktime-rbf",
-      "description": "Displays block locktime 10 and its rbf sequence semantics.",
+      "id": "locktime-sequence-fffffffd",
+      "description": "Displays block locktime 10 for transaction sequence 0xfffffffd.",
       "coin": "tbtc",
       "psbt": {
         "transaction": "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"
@@ -2435,6 +2470,7 @@
         },
         {
           "min_version": "9.26.0",
+          "max_version_exclusive": "9.27.0",
           "outcome": "success",
           "screens": [
             {
@@ -2466,6 +2502,40 @@
               "body": "confirmed"
             }
           ]
+        },
+        {
+          "min_version": "9.27.0",
+          "outcome": "success",
+          "screens": [
+            {
+              "type": "transaction_address",
+              "amount": "0.20000000 TBTC",
+              "address": "tb1p ff8v kq80 pu2c gtu7 ttga d2zn w62v 2lgu hw6p trpp wns6 nrpq au2q cuz3 7d"
+            },
+            {
+              "type": "confirm",
+              "title": "",
+              "body": "Locktime on block:\n10",
+              "longtouch": false
+            },
+            {
+              "type": "transaction_fee",
+              "amount": "0.30000000 TBTC",
+              "fee": "0.10000000 TBTC",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "High fee",
+              "body": "The fee is 50.0%\nthe send amount.\nProceed?",
+              "longtouch": true
+            },
+            {
+              "type": "status",
+              "title": "Transaction",
+              "body": "confirmed"
+            }
+          ]
         }
       ],
       "expected_signatures": [
@@ -2478,8 +2548,8 @@
       ]
     },
     {
-      "id": "locktime-litecoin-non-rbf-sequence",
-      "description": "Displays a Litecoin block locktime without Bitcoin-specific RBF wording.",
+      "id": "locktime-litecoin-sequence-fffffffe",
+      "description": "Displays a Litecoin block locktime for transaction sequence 0xfffffffe.",
       "coin": "ltc",
       "psbt": {
         "transaction": "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"
@@ -2523,6 +2593,7 @@
         },
         {
           "min_version": "9.26.0",
+          "max_version_exclusive": "9.27.0",
           "outcome": "success",
           "screens": [
             {
@@ -2554,6 +2625,40 @@
               "body": "confirmed"
             }
           ]
+        },
+        {
+          "min_version": "9.27.0",
+          "outcome": "success",
+          "screens": [
+            {
+              "type": "transaction_address",
+              "amount": "0.20000000 LTC",
+              "address": "ltc1 qw50 8d6q ejxt dg4y 5r3z arva ry0c 5xw7 kgmn 4n9"
+            },
+            {
+              "type": "confirm",
+              "title": "",
+              "body": "Locktime on block:\n10",
+              "longtouch": false
+            },
+            {
+              "type": "transaction_fee",
+              "amount": "0.30000000 LTC",
+              "fee": "0.10000000 LTC",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "High fee",
+              "body": "The fee is 50.0%\nthe send amount.\nProceed?",
+              "longtouch": true
+            },
+            {
+              "type": "status",
+              "title": "Transaction",
+              "body": "confirmed"
+            }
+          ]
         }
       ],
       "expected_signatures": [
@@ -2566,8 +2671,8 @@
       ]
     },
     {
-      "id": "locktime-litecoin-rbf-sequence",
-      "description": "Displays a Litecoin block locktime without Bitcoin-specific RBF wording.",
+      "id": "locktime-litecoin-sequence-fffffffd",
+      "description": "Displays a Litecoin block locktime for transaction sequence 0xfffffffd.",
       "coin": "ltc",
       "psbt": {
         "transaction": "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"
@@ -2611,6 +2716,7 @@
         },
         {
           "min_version": "9.26.0",
+          "max_version_exclusive": "9.27.0",
           "outcome": "success",
           "screens": [
             {
@@ -2642,6 +2748,40 @@
               "body": "confirmed"
             }
           ]
+        },
+        {
+          "min_version": "9.27.0",
+          "outcome": "success",
+          "screens": [
+            {
+              "type": "transaction_address",
+              "amount": "0.20000000 LTC",
+              "address": "ltc1 qw50 8d6q ejxt dg4y 5r3z arva ry0c 5xw7 kgmn 4n9"
+            },
+            {
+              "type": "confirm",
+              "title": "",
+              "body": "Locktime on block:\n10",
+              "longtouch": false
+            },
+            {
+              "type": "transaction_fee",
+              "amount": "0.30000000 LTC",
+              "fee": "0.10000000 LTC",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "High fee",
+              "body": "The fee is 50.0%\nthe send amount.\nProceed?",
+              "longtouch": true
+            },
+            {
+              "type": "status",
+              "title": "Transaction",
+              "body": "confirmed"
+            }
+          ]
         }
       ],
       "expected_signatures": [
@@ -4150,6 +4290,126 @@
         }
       ]
     },
+    {
+      "id": "policy-timestamp-locktime",
+      "description": "Signs and finalizes a registered WSH policy whose absolute locktime is a Unix timestamp.",
+      "coin": "tbtc",
+      "psbt": {
+        "transaction": "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",
+        "options": {
+          "force_script_config": {
+            "script_config": {
+              "type": "policy",
+              "policy": "wsh(and_v(v:pk(@0/**),after(1800000000)))",
+              "keys": [
+                {
+                  "root_fingerprint": "4c00739d",
+                  "keypath": "m/48'/1'/0'/3'",
+                  "xpub": "xpub6EhivkawbkKDTyZXUfDUntyKvkx4fcniJY3RRMdcvvkKTEt7gX7CfotifBsm1iDYzLpRk9pspB6g2r2sjxVePkCScgDmkPiGUAWg2ESMKNA"
+                }
+              ]
+            },
+            "keypath": "m/48'/1'/0'/3'"
+          }
+        }
+      },
+      "expected_needs_prevtxs": true,
+      "expectations": [
+        {
+          "max_version_exclusive": "9.27.0",
+          "outcome": "invalid_input",
+          "screens": []
+        },
+        {
+          "min_version": "9.27.0",
+          "outcome": "success",
+          "screens": [
+            {
+              "type": "confirm",
+              "title": "Spend from",
+              "body": "BTC Testnet\npolicy with\n1 keys",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "Name",
+              "body": "test timestamp locktime",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "",
+              "body": "Show policy\ndetails?",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "Policy",
+              "body": "wsh(and_v(v:pk(@0/**),after(1800000000)))",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "Key 1/1",
+              "body": "This device: [4c00739d/48'/1'/0'/3']tpubDF5MSzQdK2GfjmkNvrCZzpJhFt3if1HmrAdimugmGqWDCXYpkjxHpFZYuDxYYDAnnFMLMjLkMGvij2XV8pLtHBejgGy5RvNW4875nFGBDWv",
+              "longtouch": false
+            },
+            {
+              "type": "transaction_address",
+              "amount": "0.20000000 TBTC",
+              "address": "tb1p ff8v kq80 pu2c gtu7 ttga d2zn w62v 2lgu hw6p trpp wns6 nrpq au2q cuz3 7d"
+            },
+            {
+              "type": "confirm",
+              "title": "",
+              "body": "Locktime (UTC):\n2027-01-15\n08:00:00",
+              "longtouch": false
+            },
+            {
+              "type": "transaction_fee",
+              "amount": "0.30000000 TBTC",
+              "fee": "0.10000000 TBTC",
+              "longtouch": false
+            },
+            {
+              "type": "confirm",
+              "title": "High fee",
+              "body": "The fee is 50.0%\nthe send amount.\nProceed?",
+              "longtouch": true
+            },
+            {
+              "type": "status",
+              "title": "Transaction",
+              "body": "confirmed"
+            }
+          ]
+        }
+      ],
+      "registrations": [
+        {
+          "script_config": {
+            "type": "policy",
+            "policy": "wsh(and_v(v:pk(@0/**),after(1800000000)))",
+            "keys": [
+              {
+                "root_fingerprint": "4c00739d",
+                "keypath": "m/48'/1'/0'/3'",
+                "xpub": "xpub6EhivkawbkKDTyZXUfDUntyKvkx4fcniJY3RRMdcvvkKTEt7gX7CfotifBsm1iDYzLpRk9pspB6g2r2sjxVePkCScgDmkPiGUAWg2ESMKNA"
+              }
+            ]
+          },
+          "name": "test timestamp locktime"
+        }
+      ],
+      "expected_signatures": [
+        {
+          "input_index": 0,
+          "kind": "ecdsa",
+          "pubkey": "033a1712b41eb509147fbbd80c3fac4af4ad0f0eaeaef36e6e08918002afd8e9b2",
+          "sighash": "all"
+        }
+      ]
+    },
     {
       "id": "policy-different-multipath-derivations",
       "description": "Signs and finalizes a registered WSH policy whose two keys use different receive and change branches.",

### src/rust/bitbox02-rust/src/hww/api/bitcoin/params.rs
@@ -15,7 +15,6 @@ pub struct Params {
     pub base58_version_p2sh: u8,
     pub bech32_hrp: &'static str,
     pub name: &'static str,
-    pub rbf_support: bool,
     pub taproot_support: bool,
 }
 
@@ -34,7 +33,6 @@ const PARAMS_BTC: Params = Params {
     base58_version_p2sh: 0x05,  // starts with 3
     bech32_hrp: "bc",
     name: "Bitcoin",
-    rbf_support: true,
     taproot_support: true,
 };
 
@@ -45,7 +43,6 @@ const PARAMS_TBTC: Params = Params {
     base58_version_p2sh: 0xc4,  // starts with 2
     bech32_hrp: "tb",
     name: "BTC Testnet",
-    rbf_support: true,
     taproot_support: true,
 };
 const PARAMS_RBTC: Params = Params {
@@ -55,7 +52,6 @@ const PARAMS_RBTC: Params = Params {
     base58_version_p2sh: 0xc4,  // starts with 2
     bech32_hrp: "bcrt",
     name: "BTC Regtest",
-    rbf_support: true,
     taproot_support: true,
 };
 
@@ -66,7 +62,6 @@ const PARAMS_LTC: Params = Params {
     base58_version_p2sh: 0x32,  // starts with M
     bech32_hrp: "ltc",
     name: "Litecoin",
-    rbf_support: false,
     taproot_support: false,
 };
 
@@ -77,7 +72,6 @@ const PARAMS_TLTC: Params = Params {
     base58_version_p2sh: 0xc4,  // starts with 2
     bech32_hrp: "tltc",
     name: "LTC Testnet",
-    rbf_support: false,
     taproot_support: false,
 };
 

### src/rust/bitbox02-rust/src/hww/api/bitcoin/signtx.rs
@@ -660,7 +660,31 @@ impl<'a> TryFrom<&'a ValidatedScriptConfigWithKeypath<'a>>
     }
 }
 
-/// Singing flow:
+/// Formats the transaction's consensus nLockTime for confirmation.
+///
+/// Timestamp locktimes are shown in UTC. Whether a locktime has passed is determined using chain
+/// state, which the device does not have, so only the exact signed value is displayed.
+fn format_locktime(locktime: u32) -> Result<String, Error> {
+    let locktime = bitcoin::absolute::LockTime::from_consensus(locktime);
+    if locktime.is_block_height() {
+        return Ok(format!(
+            "Locktime on block:\n{}",
+            locktime.to_consensus_u32()
+        ));
+    }
+
+    let tm = util::datetime::get_datetime(locktime.to_consensus_u32())
+        .map_err(|_| Error::InvalidInput)?;
+    Ok(format!(
+        "Locktime (UTC):\n{}\n{}:{}:{}",
+        tm.date(),
+        tm.hour(),
+        tm.minute(),
+        tm.second(),
+    ))
+}
+
+/// Signing flow:
 ///
 /// init
 /// for each input:
@@ -689,7 +713,7 @@ impl<'a> TryFrom<&'a ValidatedScriptConfigWithKeypath<'a>>
 /// scripts), but we only skip streaming previous transactions if all inputs are taproot, for
 /// simplicity.
 ///
-/// For each output, the recipient is confirmed. At the last output, the total out, fee, locktime/RBF
+/// For each output, the recipient is confirmed. At the last output, the total out, fee and locktime
 /// are confirmed.
 ///
 /// The inputs are signed in inputs_pass2.
@@ -715,10 +739,6 @@ async fn _process(
     let coin_params = super::params::get(coin);
     // Validate the format_unit.
     let format_unit = FormatUnit::try_from(request.format_unit)?;
-    // Currently we do not support time-based nlocktime
-    if request.locktime >= 500000000 {
-        return Err(Error::InvalidInput);
-    }
     // Currently only support version 1 or version 2 tx.
     // Version 2: https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki
     if request.version != 1 && request.version != 2 {
@@ -752,8 +772,6 @@ async fn _process(
     let mut inputs_sum_pass1: u64 = 0;
 
     let mut locktime_applies: bool = false;
-    let mut rbf: bool = false;
-
     let mut hasher_prevouts = Sha256::new();
     let mut hasher_sequence = Sha256::new();
     let mut hasher_amounts = Sha256::new();
@@ -783,9 +801,6 @@ async fn _process(
             .get(tx_input.script_config_index as usize)
             .ok_or(Error::InvalidInput)?;
         validate_input(&tx_input, coin_params, script_config_account)?;
-        if tx_input.sequence < 0xffffffff - 1 {
-            rbf = true;
-        }
         if tx_input.sequence < 0xffffffff {
             locktime_applies = true;
         }
@@ -1127,31 +1142,13 @@ async fn _process(
             .await?;
     }
 
-    // Verify locktime/rbf.
-    // A locktime of 0 will also not be verified, as it's certainly in the past and can't do any
-    // harm.
-    //
-    // This is not a security feature, the extra locktime/RBF user confirmation is skipped if the tx
-    // is not rbf or has a locktime of 0.
+    // Confirm an effective non-zero nLockTime because it can defer when the signed transaction
+    // becomes valid. nLockTime is ignored if every input uses the final sequence number.
     if request.locktime > 0 && locktime_applies {
-        // The RBF nsequence bytes are often set in conjunction with a locktime,
-        // so verify both simultaneously.
+        let body = format_locktime(request.locktime)?;
         hal.ui()
             .confirm(&ConfirmParams {
-                body: &format!(
-                    "Locktime on block:\n{}\n{}",
-                    request.locktime,
-                    if coin_params.rbf_support {
-                        if rbf {
-                            "Transaction is RBF"
-                        } else {
-                            "Transaction is not RBF"
-                        }
-                    } else {
-                        // There is no RBF in Litecoin.
-                        ""
-                    }
-                ),
+                body: &body,
                 accept_is_nextarrow: true,
                 ..Default::default()
             })
@@ -1387,6 +1384,26 @@ mod tests {
     use pb::btc_payment_request_request::{Memo, memo};
     use util::bip32::HARDENED;
 
+    #[test]
+    fn test_format_locktime() {
+        assert_eq!(
+            format_locktime(499_999_999).unwrap(),
+            "Locktime on block:\n499999999"
+        );
+        assert_eq!(
+            format_locktime(500_000_000).unwrap(),
+            "Locktime (UTC):\n1985-11-05\n00:53:20"
+        );
+        assert_eq!(
+            format_locktime(1_800_000_000).unwrap(),
+            "Locktime (UTC):\n2027-01-15\n08:00:00"
+        );
+        assert_eq!(
+            format_locktime(u32::MAX).unwrap(),
+            "Locktime (UTC):\n2106-02-07\n06:28:15"
+        );
+    }
+
     fn extract_next(response: &Response) -> &pb::BtcSignNextResponse {
         match response {
             Response::BtcSignNext(next) => next,
@@ -2646,15 +2663,6 @@ mod tests {
                 );
             }
         }
-        {
-            // test invalid locktime
-            let mut init_req_invalid = init_req_valid.clone();
-            init_req_invalid.locktime = 500000000;
-            assert_eq!(
-                process(&mut TestingHal::new(), &init_req_invalid).await,
-                Err(Error::InvalidInput)
-            );
-        }
         {
             // test invalid inputs
             let mut init_req_invalid = init_req_valid.clone();