日期计算有关问题

global function date f_dateadd (string as_type, date ad_date, integer ai);//在向指定日期加上一段时间的基础上，返回新的date值（与MSSQL的dateadd函数类似）int listring lsdate ldif ai = 0 then return ad_datechoose case lower(as_type)    case 'year','yy','yyyy' //年份        li = year(ad_date) + ai        if li < 1900 or li > 3000 then return 1900-01-01        ls = string(li) + string(ad_date, '-mm-dd')    case 'quarter','qq','q' //季度        li = month(ad_date) + 3 * ai        ls = string(year(ad_date) + int((li -1) / 12)) + string(mod(li, 12) - 6 * (sign(li) - 1), '-00') + string(ad_date, '-dd')    case 'month','mm','m' //月份        li = month(ad_date) + ai        ls = string(year(ad_date) + int((li -1) / 12)) + string(mod(li, 12) - 6 * (sign(li) - 1), '-00') + string(ad_date, '-dd')    case 'day','dd','d' //天数        return RelativeDate ( ad_date, ai )    case 'week','wk','ww' //周数        return RelativeDate ( ad_date, ai * 7 )    case else        return ad_dateend chooseif ls = '1900-01-01' then return 1900-01-01ld = date(ls)if ld = 1900-01-01 then ld = f_dateadd('dd', f_dateadd('mm', date(left(ls, 8) + '01'), 1), -1)return ldend function