dfrm <- data.frame(ID=1:2,first=as.Date(c("2016-10-01","2016-12-01"), "%Y-%m-%d"),
last=as.Date(c("2016-10-03","2016-12-02"), "%Y-%m-%d"))
fill_dates(dfrm, "first", "last")
fill_dates(dfrm, "first", "last", 2, 3)
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