Even in Excel 2003 you can use EDATE by enabling Analysis ToolPak add-in
Tools > add-ins > tick Analysis ToolPak box
....or if you can't use EDATE or you don't want to then you can add a month with this formula
=DATE(YEAR(A1),MONTH(A1)+1,DAY(A1))
but that can give some odd results, e.g. if A1 is 31st Jan
To replicate EDATE you can use this version
=MIN(DATE(YEAR(A1),MONTH(A1)+1+{1,0},DAY(A1)*{0,1}))
change just the red 1 to any other number to add more than 1 month