How to convert datetime to timestamp using C#/.NET (ignoring current timezone)

At the moment you’re calling ToUniversalTime() – just get rid of that:

private long ConvertToTimestamp(DateTime value)
    long epoch = (value.Ticks - 621355968000000000) / 10000000;
    return epoch;

Alternatively, and rather more readably IMO:

private static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

private static long ConvertToTimestamp(DateTime value)
    TimeSpan elapsedTime = value - Epoch;
    return (long) elapsedTime.TotalSeconds;

EDIT: As noted in the comments, the Kind of the DateTime you pass in isn’t taken into account when you perform subtraction. You should really pass in a value with a Kind of Utc for this to work. Unfortunately, DateTime is a bit broken in this respect – see my blog post (a rant about DateTime) for more details.

You might want to use my Noda Time date/time API instead which makes everything rather clearer, IMO.

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