As I wrote, my formula assumes you have a true date in A10.
When you enter, for example, 4/2007, Excel will parse that into 1 Apr 2007. By
the way, if you just enter 4/07, and you are using US Regional Settings
(control panel stuff), Excel will parse that as 7 Apr 2007, so you might want
to be careful how you enter a date.
In any event, with any date of a month in A10, the formula =A10-DAY(A10) will
always give a date that is the last day of the preceding month.
So the criteria argument ">"&A10-DAY(A10) will evaluate, in words, to "any date
that is greater than the last day of the preceding month".
If the date were, indeed, text, the formula would not work as written.