R: calculations involving months

Ask anyone how much time has elapsed since September last year and they’ll probably start counting on their fingers: “October, November…” and tell you “just over 9 months.”

So, when faced as I was today with a data frame (named dates) like this:

pmid1       year1    month1     pmid2      year2    month2
21355427    2010     Dec        21542215   2011     Mar
21323727    2011     Feb        21521365   2011     Jun
21297532    2011     Feb        21336080   2011     Mar
21291296    2011     Apr        21591868   2011     Jun

How to add a 7th column, with the number of months between “year1/month1” and “year2/month2”?
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