MFC 327737:

USNO and possibly others have misinterpreted the maining of the
leapseconds last-update field and incorrectly increment it when changing
the file even though the leapsecond data has not changed. For instance,
if a leapsecond file is obtained from USNO, when it expires it will not
be replaced by a newer file from other sources because it has an
incorrect later last-update (version).

This corrects r304780.

PR: 225029
Submitted by: ian