"pk" wrote:
How can I calculate 11 months and 1 day from a given date using
an excel 2003 function?

That depends. Which dates would you prefer for 11 months after the
following dates on the left?

3/31/2010 + 11mo = (a) 3/3/2011 or (b) 2/28/2011
5/31/2010 + 11mo = (a) 5/1/2011 or (b) 4/30/2011
7/31/2010 + 11mo = (a) 7/1/2011 or (b) 6/30/2011
10/31/2010 + 11mo = (a) 10/1/2011 or (b) 9/30/2011
12/31/2010 + 11mo = (a) 12/1/2011 or (b) 11/30/2011

Column (a) is the result of DATE(YEAR(A1),11+MONTH(A1),DAY(A1). Column (b)
is the result of EDATE(A1,11).

For most purposes, people prefer (b).

On the other hand, for your purposes, would it bother you that with EDATE,
11mo plus 3/28/2010, 3/29/2010 and 3/30/2010 as well as 3/31/2010 are all
2/28/2011?

That does follow US law for most purposes.

If you like column (b), then 11mo plus 1day is simply 1+EDATE(A1,11). You
might need to select the Date format explicitly after entering or editing
the formula.

If you get a #NAME error, see the EDATE help page for the remedy.