library (dplyr)
start <- as.POSIXct("2018-05-18 00:00")
tseq <- seq(from = start, length.out = 1440, by = "10 mins")
observations <- data.frame(
  Time = tseq,
  Temp = sample(10:37,1440, replace = TRUE, set.seed(seed = 10)),
  Variable1 = sample(1:200,1440, replace = TRUE, set.seed(seed = 187)),
  Variable2 = sample(300:800,1440, replace = TRUE, set.seed(seed = 333))
)

observations_short <- observations %>% filter (Time <=  as.POSIXct ("2018-05-23 00:00", tz="CET") )

observations_9to5 <- observations %>% filter (Time >=  as.POSIXct ("09:00", tz="CET") ) %>% filter (Time <=  as.POSIXct ("17:00", tz="CET") )