Changed aroundtime to a time window (feature #1400)

@ProfYaffle & @adamsutton I rebased and cleaned up my version of the patch as discussed in the old pull request.
Drop this function, use _get_s64 or _get_u64, as in existing code.

And if you really want to add a get_time/set_time, make it a thin wrapper to get/set_x64.