util: Add FormatISO8601Time function for time-only ISO format

Add a new time formatting function that matches the existing date and datetime
ISO8601 formatting functions, providing consistent time display for the
traffic graph widget tooltips and other UI elements that need time-only display.

Author: rebroad

src/util/time.cpp

@@ -75,6 +75,14 @@ void MockableSteadyClock::ClearMockTime()
 
 int64_t GetTime() { return GetTime<std::chrono::seconds>().count(); }
 
+std::string FormatISO8601Time(int64_t nTime)
+{
+    const std::chrono::sys_seconds secs{std::chrono::seconds{nTime}};
+    const auto days{std::chrono::floor<std::chrono::days>(secs)};
+    const std::chrono::hh_mm_ss hms{secs - days};
+    return strprintf("%02i:%02i:%02iZ", hms.hours().count(), hms.minutes().count(), hms.seconds().count());
+}
+
 std::string FormatISO8601DateTime(int64_t nTime)
 {
     const std::chrono::sys_seconds secs{std::chrono::seconds{nTime}};

src/util/time.h

@@ -130,6 +130,7 @@ T GetTime()
  * ISO 8601 formatting is preferred. Use the FormatISO8601{DateTime,Date}
  * helper functions if possible.
  */
+std::string FormatISO8601Time(int64_t nTime);
 std::string FormatISO8601DateTime(int64_t nTime);
 std::string FormatISO8601Date(int64_t nTime);
 std::optional<int64_t> ParseISO8601DateTime(std::string_view str);