f <- function(start_time) {
start_time <- as.POSIXct(start_time)
dt <- difftime(Sys.time(), start_time, units="secs")
# Since you only want the H:M:S, we can ignore the date...
# but you have to be careful about time-zone issues
format(.POSIXct(dt,tz="GMT"), "%H:%M:%S")
}
how to use:
time1<-Sys.time()
...
f(time1)