DateTime.Compare doesn't work as expected

Question

I'm trying to compare two DateTime objects ignoring the seconds with the below function, but it gives the wrong result even though both of the DateTime objects are have the same values. Can't figure out how to exactly get it working, any help would be highly appreciated.

public static int CompareDateTime(DateTime d1, DateTime d2)
{
    d1 = d1.AddSeconds(-1 * d1.Second);
    d2 = d2.AddSeconds(-1 * d2.Second);

    int result = DateTime.Compare(d1, d2);
    string relationship;

    if (result < 0)
        relationship = "is earlier than";
    else if (result == 0)
        relationship = "is the same time as";
    else
        relationship = "is later than";

    Console.WriteLine("{0} {1} {2}", d1, relationship, d2);

    return result;

}

Result:

3/7/2017 2:54:00 PM is later than 3/7/2017 2:54:00 PM

Answer 1

The problem here is that you aren't truncating as you expect to. The best way to truncate is to create an entirely new DateTime object from the constructor by doing the following:

d1 = new DateTime(d1.Year, d1.Month, d1.Day, d1.Hour, d1.Minute, 0);
d2 = new DateTime(d2.Year, d2.Month, d2.Day, d2.Hour, d2.Minute, 0);

This will ensure you are only comparing the data you want.

In this specific case it is the milliseconds that may have been part of the datetime that were being left by your attempted truncation.