dateDate.Ticks
should give you what you're looking for.
The value of this property represents
the number of 100-nanosecond intervals
that have elapsed since 12:00:00
midnight, January 1, 0001, which
represents DateTime.MinValue. It does
not include the number of ticks that
are attributable to leap seconds.
DateTime.Ticks
If you're really looking for the Linux Epoch time (seconds since Jan 1, 1970), the accepted answer for this question should be relevant.
But if you're actually trying to "compress" a string representation of the date into an int, you should ask yourself why aren't you just storing it as a string to begin with. If you still want to do it after that, Stecya's answer is the right one. Keep in mind it won't fit into an int, you'll have to use a long.
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