Changing number category based on the content of the cell

Is there a way to automatically change the number format based on the content of a cell? I have a column which is date of birth (Long date number category), but missing numbers are 99999, however, it automatically changes this to a date - 13/10/2173. How do I change this so when I type 99999 it is automatically text category?

You could use Conditional Formatting. Select your date range HOME>>Conditional Formatting >>New Rule>> Use formula >> =A1=99999 and format Number as TEXT

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