I have a range of cells with dates like: 1/1; 1/15; 1/31; 2/1; 2/15; 2/28...

To the left of the range a user will enter a date. I want to make sure that
the date matches a value in the range above. For example, 1/7 is invalid
whereas 2/1 is fine. I'd prefer not to use a dropdown. This hasn't been easy
for me to figure out and I'm hoping someone can help me instert the correct
validate formula.