Given a series of arrays, with each individual array containing the time of the alarm set and the sleep duration, return what time to sleep.
bedTime(["07:50", "07:50"]) ➞ ["00:00"]
// The second argument means 7 hours, 50 minutes sleep duration.
bedTime(["06:15", "10:00"], ["08:00", "10:00"], ["09:30", "10:00"]) ➞ ["20:15", "22:00", "23:30"]
// The second argument of each sub list means 10 hours sleep duration.
bedTime(["05:45", "04:00"], ["07:10", "04:30"]) ➞ ["01:45", "02:40"]
// Sleep duration can be different among the arrays.