C# Epoch DateTime conversion

Question

I need to work with epoch times in c# and I have created the following two extension methods to do so:

public static DateTime ToDateTime(this double epochTime)
{
    return new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddSeconds(epochTime);
}

public static double ToEpochTime(this DateTime dt)
{
    var t = dt - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    return t.TotalSeconds;
}

I get a failure when I run the following test:

[Fact]
public void Test_EpochTime()
{
    var dateToTest = DateTime.Now;

    var epoch = dateToTest.ToEpochTime();
    var result = epoch.ToDateTime();

    Assert.Equal(result, dateToTest);
}

The result is:

Xunit.Sdk.EqualException: 'Assert.Equal() Failure

Expected: 2020-03-02T17:43:19.1830000Z

Actual: 2020-03-02T17:43:19.1831870+00:00'

Has anyone experienced this before, where there's an issue converting between the double/DateTime?

Thanks for any pointers in advance!

Answer 1

A double only has enough precision to hold down to milliseconds. If you want more precision, you will have to use DateTime.Ticks which is a long. A long is an exact value, it won't change.

If you want milliseconds, then as @Magnetron points out, there are built-in methods for that. Above all, and this is general advice, if you need an exact value, don't use double or float types.