You might be interested at looking at Daniel J. Bernstein's implementation of TAI (temps atomique international), a 64-bit representation of time values. It can deal with second, nanosecond or attosecond precision, depending on your requirements.
cr.yp.to/libtai/tai64.html
He has a number of essays about time that are worth reading. It should be noted that djb is considered a controversial character in some circles.
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