sum with dates

How do I sum 15 years to my dataset?

original dataset
         dates        val
1    2001-01-12    1.2
2    2001-02-12    1.2
3    2001-03-12    1.2

result
         dates        val
1    2016-01-12    1.2
2    2016-02-12    1.2
3    2016-03-12    1.2