Sorting "" properly for date formatted column

Having an issue with sorting dates properly.

Formula in each cell of column P is:
=IF(O6="","",INT(O6)+120)

*have also tried
=IF(ISBLANK(O6),"",INT(O6)+120)

The issue is when filtering column P I can only sort a-z as opposed to oldest to newest

Ran test on it [ =ISTEXT(P6) ] and find that if there is a date in it result is false and if not, "" returns as True.

So the issue is that Excel sees the blank "" as text and tries to filter accordingly. Is there a solution to this or am I stuck with 1/1/1900 instead of blanks if I want to be able to filter properly?