public void Format_Should_Return_DateTime_In_Universal_Format() { // Arrange IDateTimeProvider dateTimeProvider = new UtcDateTimeProvider(); // Act string output = dateTimeProvider.Format(DateTime.UtcNow); // Assert output.Should().MatchRegex(@"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z$"); }