*试一下（看变量mis）：

sort county time1
g mis="03-2008 06-2008 09-2008 12-2008"
levelsof county, l(c)
foreach i in `c'{
  levelsof time1 if  county==`"`i'"', l(t)
  foreach j in `t'{
    replace mis=subinstr(mis, `"`j'"', "",.) if county==`"`i'"'
  }
}