[flutter_tool] Include the local timezone in analytics timestamp (#37345)

packages/flutter_tools/lib/src/base/time.dart

@@ -32,3 +32,18 @@ class _FixedTimeClock extends SystemClock {
   @override
   DateTime now() => _fixedTime;
 }
+
+/// Format time as 'yyyy-MM-dd HH:mm:ss Z' where Z is the difference between the
+/// timezone of t and UTC formatted according to RFC 822.
+String formatDateTime(DateTime t) {
+  final String sign = t.timeZoneOffset.isNegative ? '-' : '+';
+  final Duration tzOffset = t.timeZoneOffset.abs();
+  final int hoursOffset = tzOffset.inHours;
+  final int minutesOffset =
+      tzOffset.inMinutes - (Duration.minutesPerHour * hoursOffset);
+  assert(hoursOffset < 24);
+  assert(minutesOffset < 60);
+
+  String twoDigits(int n) => (n >= 10) ? '$n' : '0$n';
+  return '$t $sign${twoDigits(hoursOffset)}${twoDigits(minutesOffset)}';
+}

packages/flutter_tools/lib/src/reporting/usage.dart

@@ -223,7 +223,7 @@ class _UsageImpl implements Usage {
 
     final Map<String, String> paramsWithLocalTime = <String, String>{
       ...?parameters,
-      cdKey(CustomDimensions.localTime): systemClock.now().toString(),
+      cdKey(CustomDimensions.localTime): formatDateTime(systemClock.now()),
     };
     _analytics.sendScreenView(command, parameters: paramsWithLocalTime);
   }
@@ -240,7 +240,7 @@ class _UsageImpl implements Usage {
 
     final Map<String, String> paramsWithLocalTime = <String, String>{
       ...?parameters,
-      cdKey(CustomDimensions.localTime): systemClock.now().toString(),
+      cdKey(CustomDimensions.localTime): formatDateTime(systemClock.now()),
     };
 
     _analytics.sendEvent(category, parameter, parameters: paramsWithLocalTime);

packages/flutter_tools/test/general.shard/analytics_test.dart

@@ -212,7 +212,7 @@ void main() {
 
       final String log = fs.file('analytics.log').readAsStringSync();
       final DateTime dateTime = DateTime.fromMillisecondsSinceEpoch(kMillis);
-      expect(log.contains(dateTime.toString()), isTrue);
+      expect(log.contains(formatDateTime(dateTime)), isTrue);
     }, overrides: <Type, Generator>{
       FileSystem: () => memoryFileSystem,
       SystemClock: () => mockClock,
@@ -237,7 +237,7 @@ void main() {
 
       final String log = fs.file('analytics.log').readAsStringSync();
       final DateTime dateTime = DateTime.fromMillisecondsSinceEpoch(kMillis);
-      expect(log.contains(dateTime.toString()), isTrue);
+      expect(log.contains(formatDateTime(dateTime)), isTrue);
     }, overrides: <Type, Generator>{
       FileSystem: () => memoryFileSystem,
       SystemClock: () => mockClock,

packages/flutter_tools/test/general.shard/time_test.dart

@@ -18,4 +18,22 @@ void main() {
       expect(clock.ago(const Duration(days: 10)), DateTime(1991, 8, 13));
     });
   });
+
+  group('formatting', () {
+    test('can round-trip formatted time', () {
+      final DateTime time = DateTime(1991, 7, 31);
+      expect(time.isUtc, isFalse);
+      // formatDateTime() adds a timezone offset to DateTime.toString().
+      final String formattedTime = formatDateTime(time);
+      // If a date time string has a timezone offset, DateTime.tryParse()
+      // converts the parsed time to UTC.
+      final DateTime parsedTime = DateTime.tryParse(formattedTime);
+      expect(parsedTime, isNotNull);
+      expect(parsedTime.isUtc, isTrue);
+      // Convert the parsed time (which should be utc) to the local timezone and
+      // compare against the original time which is in the local timezone. They
+      // should be the same.
+      expect(parsedTime.toLocal(), equals(time));
+    });
+  });
 }