Ignore milliseconds when comparing two datetimes

Question

This is probably a dumb question, but I cannot seem to figure it out. I am comparing the LastWriteTime of two files, however it is always failing because the file I download

Answer 1

instead of trimming unrelevant DateTime parts via creating new DateTimes, compare only relevant parts:

public static class Extensions
{
    public static bool CompareWith(this DateTime dt1, DateTime dt2)
    {
        return
            dt1.Second == dt2.Second && // 1 of 60 match chance
            dt1.Minute == dt2.Minute && // 1 of 60 chance
            dt1.Day == dt2.Day &&       // 1 of 28-31 chance
            dt1.Hour == dt2.Hour &&     // 1 of 24 chance
            dt1.Month == dt2.Month &&   // 1 of 12 chance
            dt1.Year == dt2.Year;       // depends on dataset
    }
}

I took answer by Dean Chalk as base for performance comparison, and results are:

• CompareWith is a bit faster than TrimMilliseconds in case of equal dates

• CompareWith is a faster than dates are not equal

my perf test (run in Console project)

static void Main(string[] args)
{
    var dtOrig = new DateTime(2018, 03, 1, 10, 10, 10);
    var dtNew = dtOrig.AddMilliseconds(100);

    //// perf run for not-equal dates comparison
    //dtNew = dtNew.AddDays(1);
    //dtNew = dtNew.AddMinutes(1);

    int N = 1000000;

    bool isEqual = false;

    var sw = Stopwatch.StartNew();
    for (int i = 0; i < N; i++)
    {
        // TrimMilliseconds comes from 
        // https://stackoverflow.com/a/7029046/1506454 
        // answer by Dean Chalk
        isEqual = dtOrig.TrimMilliseconds() == dtNew.TrimMilliseconds();
    }
    var ms = sw.ElapsedMilliseconds;
    Console.WriteLine("DateTime trim: " + ms + " ms");

    sw = Stopwatch.StartNew();
    for (int i = 0; i < N; i++)
    {
        isEqual = dtOrig.CompareWith(dtNew);
    }
    ms = sw.ElapsedMilliseconds;
    Console.WriteLine("DateTime partial compare: " + ms + " ms");

    Console.ReadKey();
}