Вы можете попробовать:
library(lubridate)
data_time <- data.frame(start_time = c("19:5:00","20:33:37","20:23:00","20:12:00","20:00:39"),
stop_time = c("19:11:00","20:34:39","20:23:38","20:13:00","20:00:39"), stringsAsFactors = FALSE)
data_time$difference <- difftime(as.POSIXct(data_time$stop_time, format = "%H:%M:%S"),
as.POSIXct(data_time$start_time, format = "%H:%M:%S"),
units = "mins")
С уважением,
Алексис