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: How to add a 7th column, with the number of months between 











Zero Inflated Models and Generalized Linear Mixed Models with R.
Zuur, Saveliev, Ieno (2012).