Pre-1970 dates under Win32

Under Win32, localtime returns NULL for dates pre 1970.

A number of places in the code fail to check from a NULL return from
localtime, with none seemingly more problematic than those calls within
DetermineLocalTimeZone, which causes SEGVs in a number of tests (which
clearly pass pre 1970 dates to this function).

If we want to support pre-1970 dates, which I imagine we do, then this
function requires some serious rework under Win32. If we don't, then I'm
guessing we can just drop in some error checking for a NULL return from
localtime (as I've done on my personal source base), and modify the tests
for Win32 accordingly.

Comments?

Cheers,
Claudio
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