我正在尝试将毫秒转换为 UTC 日期时间。我已经设法将毫秒转换为本地日期时间，但无法将本地日期时间转换为 UTC 日期时间。当我使用 DateTimeOffset 时，我落后了一小时，所以我相信这是本地 DateTime？

我无法将本地时间转换为 UTC DateTime

测试类

[Fact]
    public void DateTimeCalculation_MustTakeDateTimeParameter_InmilliSecond()
    {
        //Arrange
        long startDateTimeUtc = 1626987600000; //2021-07-22 22:00:00

        //act
        DateTimeOffset calculatedTimeOffset = DateTimeCalculationHelper.DateTimeCalculation(startDateTimeUtc);
        var utcDateTime = calculatedTimeOffset.UtcDateTime.ToString(); //not working

        var t1 = calculatedTimeOffset.ToUniversalTime(); //not working

        //Assert
        string actual = calculatedTimeOffset.ToString();
        string expected = "22/07/2021 10:00:00 PM +00:00";
        Assert.Equal(expected,actual);
    }

将DateTime转换为毫秒的静态类

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