It's 2:50 right now. If you agreed to meet in 20 minutes, what time will that be? Just adding 20 to 50 gives you 70 minutes, but there's no such time as "70 minutes" on a clock. So what actually happens?
The minute hand (the long hand) goes all the way around — 60 minutes, one full circle — and comes back to where it started. At that exact moment, the hour hand (the short hand) moves ahead to the next number. That's why, the instant the minutes pass 60, the hour changes by one.
Let's add 20 minutes to 2:50. 50+20=70. Since 70 is 10 more than 60, we bundle 60 of those minutes into "1 hour" and move the hour hand ahead by one, leaving just the remaining 10 minutes on the minute hand. So 2 o'clock becomes 3 o'clock, and the answer is 3:10.
Now let's go the other way. What time was it 20 minutes before 3:10? You can't subtract 20 from 10, because 20 is bigger than 10.
When that happens, you borrow 60 minutes (1 hour) from the hour before, which is 2 o'clock. Add those 60 minutes to the 10 minutes to make 70, then subtract 20, leaving 50. And the hour drops by one, to 2 o'clock. So 20 minutes before 3:10 is 2:50.
Does this calculation look familiar? That's right — it's exactly the same idea as borrowing one ten from the tens place to make ten ones when subtracting two-digit numbers. The only difference is that here, the "borrowing unit" is 60 instead of 10.
You use this kind of calculation all the time in everyday life — figuring out how many minutes until the bus arrives, how much time is left before class ends, or how long until you need to meet someone all rely on this same idea.
On our activity page, you can use sliders to change the current time and the number of minutes that pass, and compare the results side by side on two clocks. Try making the hour change several times to get a feel for how it works.