week_difference <- function(end_date, start_date){
    as.integer(difftime(head(strptime(end_date, format = "%Y-%m-%d"), 1),
               tail(strptime(start_date, format = "%Y-%m-%d"),1), units = "weeks"),0)
}


retention_week <- funnel_workout %>% group_by(userId) %>%  select(userId, cohortId) %>%
  mutate(week_number = if(!is.na(cohortId)){week_difference(funnel_registration$cohortId, funnel_workout$cohortId)}else{print(NA)})