Weird excel behavior - entering a date

I'm entering a date in a cell (X1) in a Table. 1st row are headers.

If I enter 4/11/2022, Excel changes it to 4/11/20222

If I enter 4/10/2022, Excel doesn't change it. 

I have tried not having autocomplete on, ditto autocorrect. 

I have an DATEDIF that references $x$1 and it breaks with the 20222 (apparently 18k years into the future is too much for Excel to think about!)

Searching has so far been in vain as I keep getting results for adding years to date.