I am trying to construct a formula to return the first day of the next month:

08-01-1969 = 09-01-1969
11-09-1960 = 12-01-1960
12-01-1970 = 01-01-1971

I dont have a problem with the first two. I use the month function and
increment it by one. This works fine until i run into a date in december.
How can i do this?