With a 32-bit time_t, two checks in the FormatDateTime test case didn't work
properly so far:
1. Every time_t value can be represented by struct tm, hence the test makes no
sense on such platforms and is now disabled there similar to how it's
already done with other checks in the same function.
2. std::nextafter(2147483647, +double_limit::infinity())) results in something
like 2147483647.000000238 which simply results in the limit when cast back
to an integer type, so it didn't actually test the overflow. This is fixed
by an additional std::ceil()/std::floor().
af96e17295