How To Do The Math For The Challenge Of Keeping Task Time

Here’s one way to break it down.

I have in the last few years, developed a simpler method of keeping time for the tasks I do at my day job. I did this out of necessity because when I first started at my current place of employment, I was not in the habit of keeping my time with accuracy down to the minute. Now I am, and in this article, I will show you how to very simply and quickly calculate the difference between the start and end time on any task, to the minute.

When most of us think of keeping time for tasks at work, we think of math like this:

10:30–09:13 = 1:17

And for many of us, it can be a challenge to perform that calculation in our heads. When most people do math with time, they frame the problem like this:

We often try to do the math for timekeeping just like a familiar base 10, decimal subtraction problem. The problem above is fairly straightforward since there is no “carrying over” of the hours like in a subtraction problem. Shown below is a problem where we would carry one hour over for the minutes to solve it:

To solve the problem with straight subtraction, we’d carry 60 minutes from the number to the left of the colon to the right. The 10 would be replaced by a 9, and 17 +60 becomes 77, and then the problem would look like this:

All of this is not easy for most people to do in their heads, and even more difficult on the fly while keeping track of time and multiple tasks.

To solve the problem of timekeeping, I’ve broken the problem down to a few steps that are easy to do in my head. I think you’ll find these steps to be a lot easier than performing the math in the traditional way as shown above.

Instead of working out a problem of subtraction with time, I turn it into a problem of addition, which is a lot easier to manage in my head. Let’s look again at the first problem:

Rather than try to subtract one number representing the start time from the end time of a task, I use an analog clock in my mind. Like this one:

Back to that first problem to solve, but this time, with the clock in mind:

Instead of subtracting one time from another, I just figure out 09:13 to 10:13 for the first hour, like one hour. Then I figure the remainder of the time from 10:13 to 10:30.

The remainder is 30–13=17. But instead of using subtraction, I know that the difference between 10:30 and 10:15 is 15 and that 15–13 is 2. Once I have the remainder, it’s easy to add the remainder to the hour between 09:13 to 10:13. I can usually figure these out in a few seconds and enter it before I forget it.

What about “carrying over”? Here is the second problem again:

Instead of using subtraction, I’m using addition to solve the problem. In the problem above, I just calculate the difference between 09:45 to the top of the hour which is 15 minutes. Then I add another 15 minutes to get to 10:15 for a sum of 30 minutes. Then, I add the remainder of 2 to the half-hour deduced so far:


That’s a far easier problem for me to do in my head than carrying over the hour. If I carry over the hour as 60 minutes like in a normal subtraction problem, then I add those 60 minutes to 17 minutes in the top line. Then I subtract 45 minutes from 77 minutes to get the net time.

The process is easier if I imagine an analog clock in my head. With the clock as a reference, I’m breaking time down into quarter, half and full hour increments of 15, 30 and 60 minutes. These increments are much easier to manage in my mind than a traditional subtraction problem. Once I have the increments worked out, then all that is left is a few minutes to add the quarter, half and whole hour increments. The goal here is to err on the side of addition to solve for the net time. To me, this makes for a far quicker and easier calculation.

I think it’s also useful to use the 24-hour clock rather than the 12-hour clock. Consider the following problem:

Here, we must consider the hours and how to determine the difference between them. 1 pm is the same as 13:00 hours. We could use 1 pm, but we’d still have to add 1 to 12, and we’d still use 13 as the hour while calculating the difference. Since we’re going to do that anyway, we might as well set the format of our clocks to use the 24-hour format to make the math easier. Once converted, the problem above looks like this below:

Note that as I convert 1 pm to 13:00, I am also including a leading zero in the start time to remind myself that I’m using a 24-hour clock. This way, there is no ambiguity about AM or PM, and there is a clear line of time to follow. I also use the 24-hour clock with customers to schedule appointments so that there is no question as to what I intended for the start time of any appointment.

Note also, that the time problem above requires carrying over to solve for the net time. By using a 24-hour format, it’s easier to carry 60 minutes over for an hour that is in the afternoon, and it’s easier to subtract the start time from the end time since we’re not having to convert from 12-hour to 24-hour format. This is how it would look when we carry over an hour to complete the calculation:

Note that this is just for demonstration. I’d still want to break this problem down in steps of addition rather than subtraction. I would still say to myself, 9:45, 10:45, 11:45, 12:45 and then count three hours. then I’d add 15 minutes to get from 12:45 to 13:00. Then I’d add 25 to 15 to get 40 minutes, and then I’d add 40 minutes to 3 hours to get a net time of 3:40. With practice, this becomes a 2 or 3-second process in the mind.

There is one last point I thought I’d mention: the reference source of time. You could use your watch, your phone, the clock on the wall at work, the clock on your desk phone, or the clock displayed in the Windows Task Bar. I use the clock displayed in the taskbar.

The reasons for using the Windows clock in the taskbar are several. First, it’s easier to see and it’s always in the same place. Second, your work computer has client software for checking the time, an NTP client. “NTP” stands for Network Time Protocol. The NTP protocol allows computers across the internet to synchronize time. In most consumer and business computers, the NTP client is automatically configured when Windows (or macOS) is installed, or it is configured based on a company policy and that policy is implemented by your IT department.

Your IT department has one or more of their servers set to synchronize time with a standard NTP server that is outside of your network. Here in Utah, we use the atomic clock in Boulder, Colorado hosted by the National Institute of Standards and Technology, and the US Naval Observatory, as a reference for our time. An NTP server on your network, hosted by your IT department, will synch time with an outside source, like an atomic clock hosted by your government.

That NTP server hosted by your IT department will share its time with any other computer that sends it a query. That NTP server also sets the time for all applications running on your company network. So if you’re going to keep accurate time for your work, you want to use the time server that your company hosts so that there is no conflict with the time you keep and the time your company keeps. And that means using the little clock displayed in the corner of your screen on your taskbar as a reference for keeping time.

In closing, to calculate the difference in time from start to finish, avoid using traditional decimal-style computation and subtraction. Rather, break down the procedure for calculating net time into one or more problems of addition so that you’re not carrying 60 minutes to determine the minutes’ side of the problem. Use a 24-hour format for time to avoid confusion between morning and afternoon time calculations. Lastly, you want to use the clock on your computer for reference when you calculate your time.

If you have any trepidation about keeping and recording your time down to the minute, you might find these methods of timekeeping useful and of interest.

Write on.

Written by

Husband, father, worker, philosopher, and observer. Plumbing the depths of consciousness to find the spring of happiness. Write on.

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