It is always a good idea to keep track of your own hours, but adding and subtracting with time is a bit more difficult than dealing with regular numerals. The hours add up easily enough, but the minutes have to be dealt with differently. The arithmetic works the same way, but instead of borrowing 10 from the hours column, you will borrow 60 and add it to the minutes.
Change the hours on the time card to military time by adding 12 to any hour figure that is a p.m. time. For this conversion, 4:02 p.m. would change to 16:02.
Subtract each start time from the corresponding end time. Borrow 60 minutes from the hour side of the end time if you need to. For example, if you started work at 8:20 a.m. and clocked out at 17:10, you add 60 to the minutes portion of the end time and make the hour one less (so it turns into 16:70) and subtract 8:20 from that, yielding 8 hours and 50 minutes.
Add each of your daily work totals together to get your total for the time period. Add the hours together first, then add the minutes separately. For instance, if your work totals for the time period are 8:50, 4:30 and 6:10, the hours are 18 (8+4+6) and the minutes are 90 (50+30+10).
Divide the minutes total by 60 to convert minutes to hours. In this example, you get 1.50 (90/60).
Add the total to the left of the decimal from Step 4 to the hours total, and multiply the total to the right of the decimal by 60 to convert it back to minutes. In this example, 18+1 is 19 hours, and 0.50 x 60 is 30 minutes, so your total time worked on the time card is 19 hours and 30 minutes, or 19:30.