我手头有个数据，n个观测值，每个观测值对应一个时间，观测值是每日的股价~
我需要统计每三个月的平均股价，所给数据的对应时间形式：
（例子）
>date=sdata[,1]
>date=as.Date(date)
> date[1:100]
  [1] "1990-12-19" "1990-12-20" "1990-12-21" "1990-12-24" "1990-12-25" "1990-12-26" "1990-12-27" "1990-12-28"
  [9] "1990-12-31" "1991-01-02" "1991-01-03" "1991-01-04" "1991-01-07" "1991-01-08" "1991-01-09" "1991-01-10"
[17] "1991-01-11" "1991-01-14" "1991-01-15" "1991-01-16" "1991-01-17" "1991-01-18" "1991-01-21" "1991-01-22"
[25] "1991-01-23" "1991-01-24" "1991-01-25" "1991-01-28" "1991-01-29" "1991-01-30" "1991-01-31" "1991-02-01"
[33] "1991-02-04" "1991-02-05" "1991-02-06" "1991-02-07" "1991-02-08" "1991-02-11" "1991-02-12" "1991-02-13"
[41] "1991-02-14" "1991-02-19" "1991-02-20" "1991-02-21" "1991-02-22" "1991-02-25" "1991-02-26" "1991-02-27"
[49] "1991-02-28" "1991-03-01" "1991-03-04" "1991-03-05" "1991-03-06" "1991-03-07" "1991-03-08" "1991-03-11"
[57] "1991-03-12" "1991-03-13" "1991-03-14" "1991-03-15" "1991-03-18" "1991-03-19" "1991-03-20" "1991-03-21"
[65] "1991-03-22" "1991-03-25" "1991-03-26" "1991-03-27" "1991-03-28" "1991-03-29" "1991-04-01" "1991-04-02"
[73] "1991-04-03" "1991-04-04" "1991-04-05" "1991-04-08" "1991-04-09" "1991-04-10" "1991-04-11" "1991-04-12"
[81] "1991-04-15" "1991-04-16" "1991-04-17" "1991-04-18" "1991-04-19" "1991-04-22" "1991-04-23" "1991-04-24"
[89] "1991-04-25" "1991-04-26" "1991-04-29" "1991-04-30" "1991-05-02" "1991-05-03" "1991-05-06" "1991-05-07"
[97] "1991-05-08" "1991-05-09" "1991-05-10" "1991-05-13"
怎么使用R，算得每三个月（自然月）的值呢？
即：1991-01-01到1991-3-31
       1991-4-1到1991-6--30