eg. Fill by months: i.e. you want a range of cells to be 2006/12/15, 2007/01/15, 2007/02/15 …

1. goto your desired start cell (A2) and enter =date(2006,12,15)
2. goto next cell where you wish to increment =date(year(a2),month(a2)+1,day(a2))
3. In next cell which is in sequence copy the formula and paste