Convert Monthly totals into daily average for each month...for a long time

Is there an easy way to convert monthly totals into daily averages for each month. I have 15 yeares worth of monthly data that I need to turn into daily numbers. So, below each day in March should say 57.5

3/31/1991 1784
4/30/1991 1656
5/31/1991 1666
6/30/1991 1462
7/31/1991 1800
8/31/1991 1270
9/30/1991 1276
10/31/1991 1650
....
....