C# :: Convert From Datetime To Unix Time?

I am converting unix timestamp to datetime format like this:

static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
static readonly double MaxUnixSeconds = (DateTime.MaxValue - UnixEpoch).TotalSeconds;
public static DateTime UnixTimeStampToDateTime(double unixTimeStamp) {
return unixTimeStamp > MaxUnixSeconds
? UnixEpoch.AddMilliseconds(unixTimeStamp)
: UnixEpoch.AddSeconds(unixTimeStamp);
}

use it:

Console.WriteLine("From UNIX do datetime 1300123800440 : " + UnixTimeStampToDateTime(1300123800440));

output: 14.03.2011 5:30 PM

Now i would like to have function which convert it back to unix timestamp but unfortuently my new function is cutting down last 3 digits for instance:

public static long UnixTimestampFromDateTime(DateTime date) {
long unixTimestamp = date.Ticks - new DateTime(1970, 1, 1).Ticks;
unixTimestamp /= TimeSpan.TicksPerSecond;
return unixTimestamp;
}

after use of it i am retrieving:

Console.WriteLine(UnixTimestampFromDateTime(Convert.ToDateTime("14.03.2011 5:30 PM")));

result:

1300123800