Address MSVC warning and CID 492209 with typecast of time_t to uint32_t

yes, this is a Y2K38 (or Y2106) issue, but we should have that worked out
when upgrading to the next libmozjs.

Need something like NUMBER_TO_JSVAL() and equivalent LAZY_NUMBER macro that
can deal with >32-bit ints correctly (or just always convert to double?).