Well, you haven't said am or pm for either time so assume it's 8.40 pm to 1:15 am, or 8.40 am to 1:15 pm. Do this in chunks. 8:40 to 9 is 20 minutes. 9 to 1 is 4 hours and 1:00 to 1:15 is 15 minutes. So total hours is 4 and total minutes is 35, therefore total time is 4 hr 35 mins. If the times were 8.40 pm to 1:15 pm, or 8.40 am to 1:15 am, add on 12 hours making it 16 hr 35 mins.