Calculations

Hello, 

I am a newbie to Excel.  I am trying to add numbers based on the year of number.  Meaning I have

11.1.16       1

11.1.17       5

11.1.18       7

11.15.16    2

11.15.17    3

11.15.18     5

there is a long column of numbers, but just like that.  I want to add the numbers under xx.x.16  together.  Can anyone tell me how to do this?  I tried auto sum,but that didn't work.  I tried calculate now but that didn't do anything.....