test[,"requested_delivery_start_date"] <-
as.POSIXct(test[,"requested_delivery_start_date"], tz = "GMT", format = "%Y-%m-%d %H:%M:%OS")

for(i in c(1:nrow(input_subset))){
    test[i,"requested_delivery_start_date"] <-
    as.POSIXct(test[i,"requested_delivery_start_date"], tz = "GMT", format = "%Y-%m-%d %H:%M:%OS")
}

warnings() Warning messages:
1: In [<-.factor(*tmp*, iseq, value = structure(1528822800, class = c("POSIXct", ... : invalid factor level, NA generated
2: In [<-.factor(*tmp*, iseq, value = structure(1528768800, class = c("POSIXct", ... : invalid factor level, NA generated
3: In [<-.factor(*tmp*, iseq, value = structure(1528768800, class = c("POSIXct", ... : invalid factor level, NA generated