Currently it is possible to create a one-time only maintenance period that will end after 18-01-2038 (end of unix time epoch). In other places in frontend, such action is forbidden, for example - it is not possible to save SLA configuration if the Excluded downtime of this SLA ends after 18-01-2038.

Steps to reproduce:

1. Open configuration of any existing Maintenance
2. Add the following maintenance period:
   1. Period type = One time only
   2. Date = 2038-01-17 00:00
   3. Maintenance period length: 999 days
3. Save Maintenance configuration

Result: maintenance configuration successfully saved.
Expected: it should not be possible to save such configuration, as the created maintenance period ends after the end of the unix time epoch.