In BRFSS data the interview date variable is suppressed though the the day , month and year variable is provided separately like the following given sample.

Could anyone be kind enough how I can make a quarterly variable from this separate day, month and year variable ?

* Example generated by -dataex-. For more info, type help dataex
clear
input byte(iday imonth) int iyear
25 1 94
16 6 94
16 6 94
17 6 94
16 6 94
23 6 94
28 6 94
20 9 94
21 9 94
27 9 94
17 10 94