We have tested both the 16- and 32-bit versions of NetNAT, and have found them to be unaffected by the Y2K issue. All internal dates and times are kept in the 32-bit unix format, so are correct until 2034.
The platform on which the NetNAT is run should be tested for compliance, although the fact that NetNAT neither makes decisions based upon the day of the week nor on the year, makes these issues of little concern.
The O/S in use (such as MS-DOS 6.22) will have Y2K issues of its own, and these should be reviewed. Check Microsoft's Y2K statement for DOS 6.22 at this location.