[tz] make rearguard_tarballs fails on macOS

[Paul Eggert]

On 10/18/20 2:23 AM, Robert Elz wrote:
> If I add this (new function - though parseadate() would then just call
> it and ignore the fractional seconds in the result to generate a time_t)
> it would most likely be to generate a bintime, which allows down to 1/2^64
> of a second (whatever that is in babble-speak - zetaseconds or something).

<https://man.netbsd.org/timeval.3> says struct bintime is meant to be used in 
the kernel only.

I wonder how a nanosecond-resolution timestamp would be converted to bintime and 
back without rounding error, given that the usual tradition in timestamp 
conversion is to truncate toward minus infinity instead of rounding.

The Gnulib time-parsing function sticks with struct timespec, since that's what 
POSIXish APIs give us.