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.