time: implement and test RFC1123 timestamp string

diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index daa5b8b3..eb6057a6 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -385,6 +385,21 @@ BOOST_AUTO_TEST_CASE(util_FormatISO8601Date)
     BOOST_CHECK_EQUAL(FormatISO8601Date(1317425777), "2011-09-30");
 }
 
+
+BOOST_AUTO_TEST_CASE(util_FormatRFC1123DateTime)
+{
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(std::numeric_limits<int64_t>::max()), "");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(253402300800), "");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(253402300799), "Fri, 31 Dec 9999 23:59:59 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(253402214400), "Fri, 31 Dec 9999 00:00:00 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(1717429609), "Mon, 03 Jun 2024 15:46:49 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(0), "Thu, 01 Jan 1970 00:00:00 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(-1), "Wed, 31 Dec 1969 23:59:59 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(-1717429609), "Sat, 31 Jul 1915 08:13:11 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(-62167219200), "Sat, 01 Jan 0000 00:00:00 GMT");
+    BOOST_CHECK_EQUAL(FormatRFC1123DateTime(-62167219201), "");
+}
+
 BOOST_AUTO_TEST_CASE(util_FormatMoney)
 {
     BOOST_CHECK_EQUAL(FormatMoney(0), "0.00");
diff --git a/src/util/time.cpp b/src/util/time.cpp
index 68af84a8..9e0715e5 100644
--- a/src/util/time.cpp
+++ b/src/util/time.cpp
@@ -10,6 +10,7 @@
 #include <util/check.h>
 #include <util/strencodings.h>
 
+#include <array>
 #include <atomic>
 #include <chrono>
 #include <optional>
@@ -17,6 +18,9 @@
 #include <string_view>
 #include <thread>
 
+static constexpr std::array<std::string_view, 7> weekdays{"Thu", "Fri", "Sat", "Sun", "Mon", "Tue", "Wed"}; // 1970-01-01 was a Thursday.
+static constexpr std::array<std::string_view, 12> months{"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
+
 void UninterruptibleSleep(const std::chrono::microseconds& n) { std::this_thread::sleep_for(n); }
 
 static std::atomic<std::chrono::seconds> g_mock_time{}; //!< For testing
@@ -117,6 +121,24 @@ std::optional<int64_t> ParseISO8601DateTime(std::string_view str)
     return int64_t{TicksSinceEpoch<std::chrono::seconds>(tp)};
 }
 
+std::string FormatRFC1123DateTime(int64_t time)
+{
+    if (time < -62167219200 || 253402300799 < time) {
+        // 4-digit year, so only support years 0 to 9999
+        return "";
+    }
+    const std::chrono::sys_seconds secs{std::chrono::seconds{time}};
+    const auto days{std::chrono::floor<std::chrono::days>(secs)};
+    const auto w{days.time_since_epoch().count() % 7}; // will be in the range [-6, 6]
+    std::string_view weekday{weekdays.at(w >= 0 ? w : w + 7)};
+    const std::chrono::year_month_day ymd{days};
+    std::string_view month{months.at(unsigned{ymd.month()} - 1)};
+    const std::chrono::hh_mm_ss hms{secs - days};
+    // examples: Mon, 27 Jul 2009 12:28:53 GMT
+    //           Fri, 31 May 2024 19:18:04 GMT
+    return strprintf("%03s, %02u %03s %04i %02i:%02i:%02i GMT", weekday, unsigned{ymd.day()}, month, signed{ymd.year()}, hms.hours().count(), hms.minutes().count(), hms.seconds().count());
+}
+
 struct timeval MillisToTimeval(int64_t nTimeout)
 {
     struct timeval timeout;
diff --git a/src/util/time.h b/src/util/time.h
index 655db447..0f98726d 100644
--- a/src/util/time.h
+++ b/src/util/time.h
@@ -136,6 +136,12 @@ std::string FormatISO8601DateTime(int64_t nTime);
 std::string FormatISO8601Date(int64_t nTime);
 std::optional<int64_t> ParseISO8601DateTime(std::string_view str);
 
+/**
+ * RFC1123 formatting https://www.rfc-editor.org/rfc/rfc1123#section-5.2.14
+ * Used in HTTP/1.1 responses
+ */
+std::string FormatRFC1123DateTime(int64_t nTime);
+
 /**
  * Convert milliseconds to a struct timeval for e.g. select.
  */