题主应该是想先把13年时time=1的个体先筛选出来？我假设你代表个体的变量是id：
gen t1 = ( time == 1 & year == 2013 )
bys id: gen want = sum(t1)
gen policy = year - 2013 if ( want == 1 & year <= 2013)
drop t1 want

gen time = 0
gen policy = .

foreach year in 2013/. {
    if `year\' == 2013 {
        gen time = 1
    } else {
        gen policy = `year\' - 2013
    }
}