library(lubridate)
#a character string representing a datetime
(dt_char <- "2021-04-30T20:30:00.000Z")
#a numeric representation of the above datetime suitable for calculations
(dt_1 <- lubridate::parse_date_time(dt_char,"ymd HMS"))
#a recasting of the above numeric datetime to some character string of desired format
(dt_char2 <- format(dt_1,"%d/%m/%y %H:%M:%OS"))