summing of two dates

hey

i have a question regarding the summing of two dates

i have

A1: 20 Days 19:46:49 Hours
A2: 12 Days 01:30:19 Hours
B1: =A1+A2 = 01 Days 21:17:08 Hours

All 3 cells with this formating: TT "Days" hh:mm:ss "Hours"

A1 and A2 are results of calculating formulas.

now the first problem is obvious. the total (B1) should be 1 month 01 Days 21:17:08 Hours.
this is achieved by changing the formating to: MM "Months" TT "Days" hh:mm:ss "Hours

now the problem i have is that A1 and A2 both count 1 month if i format them to MM "Months" TT "Days" hh:mm:ss "Hours
That means with this formating i have:


A1: 01 Months 20 Days 19:46:49 Hours
A2: 01 Months 12 Days 01:30:19 Hours
B1: =A1+A2 = 02 Months 01 Days 21:17:08 Hours

that is not correct! why is excel giving them one month even if there is no month in the calculation result..?

The correct result would be:
A1: 00 Months 20 Days 19:46:49 Hours
A2: 00 Months 12 Days 01:30:19 Hours
B1: =A1+A2 = 01 Months 01 Days 21:17:08 Hours

hope you can help me..

thanks