Calculate values on a Bex. Variable

I have two variables on my query. 0calmonth and

zdate.

my requirement is after I enter value to ocalmonth variable, value must be calculated and displayed automatically on zdate variable. (Last day of the month from the entered month on 0calmonth).

Tried writing enhancement for this. I am unable to pass the value(month) entered on 0calmonth variable to zdate variable.

Any idea on how to do this, would be of great help.

Thanks