One slightly old-fashioned way with your data in a data.frame or tibble called dat1
library(lubridate)
# if needed convert the datetime column to a time
dat1[, 1] <- dmy_hm(dat1[, 1])
subset(dat1, datetime >= dmy_hm("14/04/2017 17:30") & datetime <= dmy_hm("15/05/2017 02:00"))
subset(dat1, datetime >= dmy_hm("14/04/2017 17:30") & datetime <= dmy_hm("15/05/2017 02:00"))