date from a string and int value in ssis

hi so my table is as follows

active      expirymonth     expiryyear

0              oct                         2013

0              feb                            2015

1               jan                        2012
and so on

now my requirement is that if active =0 then in a new table lets say LASTDATE and they want a date with the last day of the expiry month and year  and if active is anything else write NULL for eg

LASTDATE

10/31/2013

02/28/2015

NULL

I went to derived tables and made a new table called lastdate. What expression do i build in the expression builder to get the required result as i could not find a way to convert the text into int and then into a date