Calculate elapsed months.

calc_elapsed_months.r

library(dplyr)
library(lubridate)

df <- data_frame(
  id = c(1, 1, 1, 2, 2, 2),
  ym = c("201512", "201601", "201603", "201512", "201602", "201603")
)

elapsed_months <- function(end, start) {
  12 * (year(end) - year(start)) + (month(end) - month(start))
}

df %>%
  mutate(ymd = ymd(paste0(ym, "01"))) %>% 
  group_by(id) %>%
  mutate(
    lag = elapsed_months(ymd, lag(ymd)),
    ymd = NULL
  )
# Source: local data frame [6 x 3]
# Groups: id [2]
# 
# id     ym   lag
# <dbl>  <chr> <dbl>
# 1     1 201512    NA
# 2     1 201601     1
# 3     1 201603     2
# 4     2 201512    NA
# 5     2 201602     2
# 6     2 201603     1