Fix time zone issues in 32bit builds

The bug is obvious, tracking it down wasn't.
A simple thinko brought us time_t instead of timestamp_t - which doesn't
matter on 64bit builds, but makes things fail in weird ways on 32bit
builds - like our Windows binaries.
In this case instead of checking today's date to detect the timezone
offset we happened to be using a date in December 1969, so no dailight
savings, so the times were off.

Fixes #655
Fixes #667
See #670

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>