How to test in xUnit, UTC Date in millisecond for API method

Question

I am working on xUnit test for Web API project. One of the method takes UTC Date in millisecond, how can I test which confirms the input parameter to API method is valid millisecond. I have logic to convert millisecond to date

public static DateTimeOffset DateTimeCalculation(long milSec)
{
        DateTimeOffset epochTime = new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero);
        return epochTime.AddMilliseconds(milSec);
}

test method

   [Fact]
    public async Task GetPersonMethod_MustTake_DateParameter_InMilliSecond()
    {
        //Arrange
        var fixture = new Fixture();
        long startDateTimeUtc = 1626994800000;


        DateTimeOffset starTimeOffset = DateTimeCalculationHelper.DateTimeCalculation(startDateTimeUtc);

        //Act



        //Assert
    }

Answer 1

When writing unit tests, you should have a clear understanding about actual result and expected result. Your test should look like below.

Alway's ask yourself, Given some input what's the expected output?

public void GetPersonMethod_MustTake_DateParameter_InMilliSecond()
    {
        //Arrange
        long startDateTimeUtc = 1626994800000;

        //Act
        DateTimeOffset starTimeOffset = DateTimeCalculationHelper.DateTimeCalculation(startDateTimeUtc);

        //Assert
        string actual = starTimeOffset.ToString();
        string expected = "22/07/2021 11:00:00 PM +00:00";
        Assert.Equal(expected, actual);
    }