[henrikschroder]

2011-02-28 + 1m1d = (2011-02-28 + 1m) + 1d = 2011-03-28 + 1d = 2011-03-29

2011-02-28 + 1d1m = (2011-02-28 + 1d) + 1m = 2011-03-01 + 1m = 2011-04-01

Also, you have these overflow rules:

2011-03-31 + 1m = 2011-04-30

2011-04-30 - 1m = 2011-03-30