test [nfc]: Generate timestamps

Author: PIG208

test/example_data.dart

@@ -68,6 +68,20 @@ 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.
 //
@@ -469,7 +483,7 @@ StreamMessage streamMessage({
     'last_edit_timestamp': lastEditTimestamp,
     'subject': topic ?? _defaultTopic,
     'submessages': submessages ?? [],
-    'timestamp': timestamp ?? 1678139636,
+    'timestamp': timestamp ?? utcTimestamp(),
     'type': 'stream',
   }) as Map<String, dynamic>);
 }
@@ -510,7 +524,7 @@ DmMessage dmMessage({
     'last_edit_timestamp': lastEditTimestamp,
     'subject': '',
     'submessages': submessages ?? [],
-    'timestamp': timestamp ?? 1678139636,
+    'timestamp': timestamp ?? utcTimestamp(),
     'type': 'private',
   }) as Map<String, dynamic>);
 }
@@ -659,7 +673,7 @@ UpdateMessageEvent updateMessageEditEvent(
     messageId: messageId,
     messageIds: [messageId],
     flags: flags ?? origMessage.flags,
-    editTimestamp: editTimestamp ?? 1234567890, // TODO generate timestamp
+    editTimestamp: editTimestamp ?? utcTimestamp(),
     moveData: null,
     origContent: 'some probably-mismatched old Markdown',
     origRenderedContent: origMessage.content,
@@ -690,7 +704,7 @@ UpdateMessageEvent _updateMessageMoveEvent(
     messageId: messageIds.first,
     messageIds: messageIds,
     flags: flags,
-    editTimestamp: 1234567890, // TODO generate timestamp
+    editTimestamp: utcTimestamp(),
     moveData: UpdateMessageMoveData(
       origStreamId: origStreamId,
       newStreamId: newStreamId ?? origStreamId,