Here is how to easily fix it. Go to the Plugins -> Plugin Editor then select Date Calculator. You are now on editable date-calculator.php
FOR date_add
There you need to find line 203 and replace with this code:
“$datecalc = date($format, mktime(0, 0, 0, date(“m”)+$month, date(“d”)+$day, date(“Y”)+$year));” WITHOUT QUOTE
FOR date_sub
Find line 244 and replace with this code:
“$datecalc = date($format, mktime(0, 0, 0, date(“m”)-$month, date(“d”)-$day, date(“Y”)-$year));” WITHOUT QUOTE
Now you can use custom date format.
