Calculate Head Count over Month based on start / end date.

head_count.R

library(tidyverse)
library(lubridate)

dat <- tribble(~id,      ~start,     ~end,  
               0102,       "2018-01-02",   "2018-01-07",   
               2190,       "2018-01-01",   "2018-03-06", 
               2432,       "2018-01-03",   "2018-05-07",  
               0121,       "2018-01-03",   "2018-02-04",  
               0121,       "2018-01-02",   "2018-01-15", 
)


tibble(data = list(dat)) %>% 
  crossing(month = seq(min(as.Date(dat$start)),
                       max(as.Date(dat$end)),
                       by = "+1 month")) %>%
  rowwise(month) %>% 
  mutate(filter(data,
                floor_date(as.Date(start), "months") <= .env$month,
                as.Date(end) > .env$month) %>% 
                count)