Unknown Start and End dates

Dates are captured in format DD-MMM-YY.

If either are unknown, entered as follows-

Day=UN, month=UNK, year=UNKN or 0000;

example: UN APR 2021.

Looking for 2 formulas for following outputs-

1. If start date is unknown, need output as the first day of the first month of year 1900.

Example: UN MAR 2022 should output as 01-MAR-22.

1. If end date is unknown, need output as last day of the last month of year 2025.

Example: UN UNK 2023 should output as 31-DEC-23.