Index: binaries/data/mods/public/gui/common/datetime.js
===================================================================
--- /dev/null
+++ binaries/data/mods/public/gui/common/datetime.js
@@ -0,0 +1,20 @@
+function timeOfDayToString(time, showSeconds = false)
+{
+	return showSeconds ?
+		// Translation: Format of the local time as shown for example in the realtime overlay,
+		// chat in the lobby, game setup, and session, and the lobby game start time.
+		// For a list of symbols that you can use, see:
+		// http://userguide.icu-project.org/formatparse/datetime#TOC-Date-Field-Symbol-Table
+		Engine.FormatMillisecondsIntoDateStringLocal(time, translateWithContext("time of day, with seconds", "HH:mm:ss")) :
+		// Translation: Format of the local time as shown for example in the realtime overlay,
+		// chat in the lobby, game setup, and session, and the lobby game start time.
+		// For a list of symbols that you can use, see:
+		// http://userguide.icu-project.org/formatparse/datetime#TOC-Date-Field-Symbol-Table
+		Engine.FormatMillisecondsIntoDateStringLocal(time, translateWithContext("time of day, without seconds", "HH:mm"));
+}
+
+function chatTimestampToString(time)
+{
+	// Translation: Time prefix as shown for example in the chat in the lobby, game setup, and session.
+	return sprintf(escapeText(translate("[%(time)s]")), { "time": timeOfDayToString(time) });
+}
Index: binaries/data/mods/public/gui/common/functions_global_object.js
===================================================================
--- binaries/data/mods/public/gui/common/functions_global_object.js
+++ binaries/data/mods/public/gui/common/functions_global_object.js
@@ -70,7 +70,7 @@
 		counters.push(sprintf(translate("FPS: %(fps)4s"), { "fps": Engine.GetFPS() }));
 
 	if (Engine.ConfigDB_GetValue("user", "overlay.realtime") === "true")
-		counters.push((new Date()).toLocaleTimeString());
+		counters.push(timeOfDayToString(Date.now(), true));
 
 	// If game has been started
 	if (typeof appendSessionCounters != "undefined")
Index: binaries/data/mods/public/gui/gamesetup/gamesetup.js
===================================================================
--- binaries/data/mods/public/gui/gamesetup/gamesetup.js
+++ binaries/data/mods/public/gui/gamesetup/gamesetup.js
@@ -2268,9 +2268,7 @@
 
 	if (Engine.ConfigDB_GetValue("user", "chat.timestamp") == "true")
 		text = sprintf(translate("%(time)s %(message)s"), {
-			"time": sprintf(translate("\\[%(time)s]"), {
-				"time": Engine.FormatMillisecondsIntoDateStringLocal(Date.now(), translate("HH:mm"))
-			}),
+			"time": chatTimestampToString(Date.now()),
 			"message": text
 		});
 
Index: binaries/data/mods/public/gui/lobby/lobby.js
===================================================================
--- binaries/data/mods/public/gui/lobby/lobby.js
+++ binaries/data/mods/public/gui/lobby/lobby.js
@@ -1036,9 +1036,8 @@
 	sgGameStartTime.hidden = !game.startTime;
 	if (game.startTime)
 		sgGameStartTime.caption = sprintf(
-			// Translation: %(time)s is the hour and minute here.
 			translate("Game started at %(time)s"), {
-				"time": Engine.FormatMillisecondsIntoDateStringLocal(+game.startTime * 1000, translate("HH:mm"))
+				"time": timeOfDayToString(+game.startTime * 1000)
 			});
 
 	sgNbPlayers.caption = sprintf(
@@ -1397,19 +1396,9 @@
 	if (Engine.ConfigDB_GetValue("user", "chat.timestamp") != "true")
 		return formattedMessage;
 
-	// Translation: Time as shown in the multiplayer lobby (when you enable it in the options page).
-	// For a list of symbols that you can use, see:
-	// https://sites.google.com/site/icuprojectuserguide/formatparse/datetime?pli=1#TOC-Date-Field-Symbol-Table
-	let timeString = Engine.FormatMillisecondsIntoDateStringLocal(msg.time ? msg.time * 1000 : Date.now(), translate("HH:mm"));
-
-	// Translation: Time prefix as shown in the multiplayer lobby (when you enable it in the options page).
-	let timePrefixString = sprintf(translate("\\[%(time)s]"), {
-		"time": timeString
-	});
-
 	// Translation: IRC message format when there is a time prefix.
 	return sprintf(translate("%(time)s %(message)s"), {
-		"time": senderFont(timePrefixString),
+		"time": senderFont(chatTimestampToString(msg.time ? msg.time * 1000 : Date.now())),
 		"message": formattedMessage
 	});
 }
Index: binaries/data/mods/public/gui/session/messages.js
===================================================================
--- binaries/data/mods/public/gui/session/messages.js
+++ binaries/data/mods/public/gui/session/messages.js
@@ -901,9 +901,7 @@
 	// Save to chat history
 	let historical = {
 		"txt": formatted,
-		"timePrefix": sprintf(translate("\\[%(time)s]"), {
-			"time": Engine.FormatMillisecondsIntoDateStringLocal(Date.now(), translate("HH:mm"))
-		}),
+		"timePrefix": chatTimestampToString(Date.now()),
 		"filter": {}
 	};