Some DEC code for OpenVMS had been converting variables containing times using the Unix epoch date to and from the OpenVMS-format dates* using the LIB$ LIBRTL calls, and the LIBRTL calls had a documented format limit of 9,999 days.
When 19-May-1997 rolled around, you needed to use a different conversion sequence for your Unix epoch dates, or to have applied the then-available LIBRTL 10K Delta Time patch that extended the permitted day field.
More to the OP's point, the longest continuous OpenVMS server uptime I'm aware of was around seventeen years.
The downside of that being seventeen years of unapplied patches.
*VMS uses 17-Nov-1958 as its base date; that date was chosen to match the date commonly used by the Smithsonian Astrophysical Observatory
When 19-May-1997 rolled around, you needed to use a different conversion sequence for your Unix epoch dates, or to have applied the then-available LIBRTL 10K Delta Time patch that extended the permitted day field.
More to the OP's point, the longest continuous OpenVMS server uptime I'm aware of was around seventeen years.
The downside of that being seventeen years of unapplied patches.
*VMS uses 17-Nov-1958 as its base date; that date was chosen to match the date commonly used by the Smithsonian Astrophysical Observatory