test [nfc]: Add utcTimestamp

Author: PIG208

test/example_data.dart

@@ -69,6 +69,20 @@ ZulipApiException apiExceptionUnauthorized({String routeName = 'someRoute'}) {
     data: {}, message: 'Invalid API key');
 }
 
+////////////////////////////////////////////////////////////////
+// Time values.
+//
+
+final timeInPast = DateTime.utc(2025, 4, 1, 8, 30, 0);
+
+/// The UNIX timestamp, in UTC seconds.
+///
+/// This is the commonly used format in the Zulip API for timestamps.
+int utcTimestamp([DateTime? dateTime]) {
+  dateTime ??= timeInPast;
+  return dateTime.toUtc().millisecondsSinceEpoch ~/ 1000;
+}
+
 ////////////////////////////////////////////////////////////////
 // Realm-wide (or server-wide) metadata.
 //