Derivation
variable: intdate [interview date] (table[s]: tb_tbx, tb_tbxto2007, tbold_tbpart1/2, tb_tbo, tb_tboto2007, tbold_control)
variable: firstart_date [first ART date] (table[s]: hivart_kps_known_all)
variable: artstatus [ART status] (table[s]: derived variable)
gen arvmths=int(datehiv-firstart_date)/30.5 if artstatus==1