GPS tag is magic enough for autodelete.
Less restrictive date range on 32bit.

Using the system time functions (with time_t) tends to limit dates to
1970+-68 on 32bit platforms. Not using them of course fixes this, and also
gives us the same behaviour for dates on all platforms (I doubt things like
"day 0" actually work everywhere).

I should probably drop struct tm too, because it's terrible, but it seemed
safer to just reimplement mktime and gmtime. I will probably change the
data structure later.