|A formula can not change the value of another cell.|
So if A1 = 03/01/2015 then a formula in B1 can not change the year in cell A1.
You can use a formula to change the Date with a formula,
but the result will be in another cell, like B1
If you Date is in cell A1
then in cell B1 enter the formula:
Cell B1 will have the new modified date.
Will that work for you?
Also, for future reference, Excel questions are best asked in the Office Software Forum.