libtai supports two time scales: (1) TAI64, covering a few hundred
billion years with 1-second precision; (2) TAI64NA, covering the same
period with 1-attosecond precision. Both scales are defined in terms of
TAI, the current international real time standard.

libtai provides an internal format for TAI64, struct tai, designed for
fast time manipulations. The tai_pack() and tai_unpack() routines
convert between struct tai and a portable 8-byte TAI64 storage format.
libtai provides similar internal and external formats for TAI64NA.

libtai provides struct caldate to store dates in year-month-day form. It
can convert struct caldate, under the Gregorian calendar, to a modified
Julian day number for easy date arithmetic.

This version of libtai requires a UNIX system with gettimeofday(). It
will be easy to port to other operating systems with compilers
supporting 64-bit arithmetic.

WWW: http://cr.yp.to/libtai.html