If you want to deal with date/time properly, then you will need a separate field for nanoseconds anyways, in order to deal with leap seconds; in such a case the

If you want to deal with date/time properly, then you will need a separate field for nanoseconds anyways, in order to deal with leap seconds; in such a case the number of nanoseconds can exceed one billion. In such a case the nanoseconds can fit in 32-bits and you don't need 64-bits; only the seconds is 64-bits and nanoseconds is 32-bits.