If you are “not able to apply a formula to a custom format,” then this immediately suggests to me that Excel no longer considers the contents of that cell to be a number:   it is now considered to be a string.   And that, therefore, would be the actual root cause of your problem.   You must be certain that you are, in fact, posting a date value, and then (if necessary) instructing Excel to format the cell (range) as you desire.

Here’s an easy way to check.   In Excel, load the spreadsheet and then change the (date) formatting, say of the column.   Values that truly are “dates” will be reformatted.   Values that are strings, of course, will not.   String-valued columns will allow any string value (including those representing non-existent dates like February 30th).