Calculating the hours minute secs and millseconds for few groups - Error on conversion

General
1

Hi Team,
I have a column which has date,time (1/8/2018 12:24:57 AM) with one more column ID. i have to calculate the total number of hours between the time period group by ID. Below is the code i am trying

start.time is the column

foo<-real %>% mutate(time_of_day=hms::hms(second(real$start.time),minute(real$start.time),hour(real$start.time)))

The error i am getting is

Error in mutate_impl(.data, dots) :
Evaluation error: character string is not in a standard unambiguous format.

Thanks for the help.