main_test.dart

// https://pub.dev/packages/clock
import 'package:clock/clock.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:hooks_riverpod/hooks_riverpod.dart';
import 'package:intl/intl.dart';

final clockProvider = Provider((ref) => const Clock());
final someTimeService = Provider(SomeTimeService.new);

class SomeTimeService {
  SomeTimeService(this._ref);

  final Ref _ref;
  static final _format = DateFormat();
  String get formattedNow => _format.format(_ref.read(clockProvider).now());
}

void main() {
  test('today Clock.fixed()', () async {
    final container = ProviderContainer(
      overrides: [
        clockProvider.overrideWithValue(Clock.fixed(DateTime(1986, 9, 26))),
      ],
    );
    final service = container.read(someTimeService);
    expect(service.formattedNow, 'September 26, 1986 12:00:00 AM');
    await Future<void>.delayed(const Duration(seconds: 1));
    expect(service.formattedNow, 'September 26, 1986 12:00:00 AM');
  });
  test('today Clock(() => ...)', () {
    var now = DateTime(1986, 9, 26);
    final container = ProviderContainer(
      overrides: [
        clockProvider.overrideWithValue(Clock(() => now)),
      ],
    );
    final service = container.read(someTimeService);
    expect(service.formattedNow, 'September 26, 1986 12:00:00 AM');
    now = DateTime(2021, 8, 31);
    expect(service.formattedNow, 'August 31, 2021 12:00:00 AM');
  });
}