My data are from 2008 to 2010, with repeated measures for same subjects. I
wish to compute number of months since january 2008.

The data are like the following:

ID date
1 4/12/2008
1 4/13/2008
1 5/11/2008
2 3/21/2009
2 4/22/2009
2 8/05/2009
...

the date column are in the format "%m/%d/%y". i wish to obtain

ID month
1 4
1 4
1 5
2 15
2 16
2 20
...

also, for the same ID with two identical month, I only want to keep the
last one. can any expert help with this question?

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