Nested if and Left and right statements

hi guys new to this forum,

trying to write an excel nested if

here is the situation i have mm/year in A1.. 1/2011

from a1 to a4 i want to write out if A1 left "1" then make a1= 2, if its 2 then make a2 = 3

thats for changing month then i need another iffor the year.. the value of a1 left is 4 then a2 will change to 01/2012 and a3 will 3/2012

i was starting out with simple left but with 2 nested if's i get an error..
=IF(LEFT(A1)="1",LEFT(A1 +1),A1,IF(LEFT(A1)="2",LEFT(A1+1),A1))

you entered too many arguments for this function..

any ideas..