1. library(tidyverse)
    2. library(lubridate)
    3. set.seed(1)
    4. df1 = tibble(
    5.   ID = rep(1:3, each = 10),
    6.   Time = rep(ymd("2013-01-11") + 1:10, 3),
    7.   PM25 = sample(300, 30))
    8. df2 = tibble(
    9.   ID = c(1,3),
    10.   T1 = ymd(c("2013-01-17","2013-01-13")),
    11.   T2 = ymd("2013-01-19") + 1:2)
    12. df1
    13. df2
    15. df1 %>%  group_nest(ID) %>% right_join(df2,by="ID") %>% 
    16.   mutate(pm25_m=
    17.  pmap_dbl(.[-1],~filter(
    18.    ..1,between(Time,..2,..3))
    19.   |> pull(PM25)|> mean()))