r - Comparing dates inside group using same reference -

i have data table different patients ("spell") , several temperature ("temp") measures each patient ("episode"). have date , time in each temperature taken.

spell episode         date    temp  1       3       2-1-17 21:00   40  1       2       2-1-17 20:00   36  1       1       1-1-17 10:00   37  2       3       2-1-17 15:00   36  2       2       2-1-17 10:00   37  2       1       1-1-17 8:00    36  3       1       3-1-17 10:00   40  4       3       4-1-17 15:00   36  4       2       3-1-17 12:00   40  4       1       3-1-17 10:00   39  5       7       3-1-17 17:30   36  5       6       2-1-17 17:00   36  5       5       2-1-17 16:00   37  5       1       1-1-17 9:00    36  5       4       1-1-17 14:00   39  5       3       1-1-17 13:00   40  5       2       1-1-17 11:00   39 

i interested in keeping measurements done 24h prior last one, have grouped observations spell , reverse date, unsure on how in-group comparison using same reference (in case, first row each group). result should be:

    spell episode         date    temp  1       3       2-1-17 21:00   40  1       2       2-1-17 20:00   36  2       3       2-1-17 15:00   36  2       2       2-1-17 10:00   37  3       1       3-1-17 10:00   40  4       3       4-1-17 15:00   36  5       7       3-1-17 17:30   36 

would appreciate ideas point me right direction.

edit: date in d-m-yy h:m format. here's dput data:

structure(list(spell = c(1l, 1l, 1l, 2l, 2l, 2l, 3l, 4l, 4l,  4l, 5l, 5l, 5l, 5l, 5l, 5l, 5l), episode = c(3l, 2l, 1l, 3l,  2l, 1l, 1l, 3l, 2l, 1l, 7l, 6l, 5l, 1l, 4l, 3l, 2l), date = c("2-1-17 21:00",  "2-1-17 20:00", "1-1-17 10:00", "2-1-17 15:00", "2-1-17 10:00",  "1-1-17 8:00", "3-1-17 10:00", "4-1-17 15:00", "3-1-17 12:00",  "3-1-17 10:00", "3-1-17 17:30", "2-1-17 17:00", "2-1-17 16:00",  "1-1-17 9:00", "1-1-17 14:00", "1-1-17 13:00", "1-1-17 11:00" ), temp = c(40l, 36l, 37l, 36l, 37l, 36l, 40l, 36l, 40l, 39l,  36l, 36l, 37l, 36l, 39l, 40l, 39l)), .names = c("spell", "episode",  "date", "temp"), class = c("data.table", "data.frame"), row.names = c(na,  -17l), .internal.selfref = <pointer: 0x00000000001f0788>) 

library(dplyr)  df %>%    mutate(date2 = as.numeric(strptime(df$date, "%d-%m-%y %h:%m"))) %>%    group_by(spell) %>%    filter(date2 >= (max(date2) - 60*60*24)) %>%   select(-date2)