Okay tip number one is going to be use a time calculation app instead of relying on hacks like this. This is only for when apps aren’t available and you have to rely on your own “skill” and “knowledge”.
But sometimes that’s all you’ve got so here we go.
This is also not for when you are programming a time library. (Where instead you should convert to seconds, subtract, then convert back.) This is for back-of-the-envelope human calculation.
I am assuming both times are within 24 hours of each other. If not, we’re gonna need further hacks. That’s gonna have to wait to lesson two.
Then, if one of the times is AM and the other is PM, or vice versa, add 1200 to the first time. We’re going to use the running example of 6:49 PM as the start time and 4:20 AM as the ending time. So erase those colons (we’ll put them back in last step) and add 1200 to the later time. So in our example that gives us 649 to 1620.
Now, some countries use a “24 hour clock” where the afternoon always has 1200 added and the morning never has. If the times cross noon that system is gonna be great here but if it’s midnight they’re crossing, it’s not applicable. Here, we’re always adding 1200 to the latter time for each time it crossed midnight or noon. So 1 pm to 2 pm? Don’t add anything. 1 pm to 2 am? Add 1200 (so we’re comparing 100 to 1400; with this hack, it’s 1400 even though it’s 2 am). 2pm to 1pm the following day, crossing both midnight and noon? Add the 1200 twice, so we’re comparing 200 to 2500.
Now if the minutes hand are the same or lower on the earlier time stamp, you can skip a step. 2500 is 2300 later than 200, i.e. it’s 23h and 0 minutes later.
But if it’s not, like in our 649 to 1620 example where the earlier time has a longer minute hand? Subtract forty from it, in this case turning 1620 to 1580. (We’re adding sixty and subtracting a hundred, if that’s easier do that, if subtracting 40 in one step is easier do that instead.)
So now we have 649 to 1580. Just subtract. In this example that makes 931. Now, that is not 931 minutes. Instead, it’s time to add the colon back in (or keep it in all along). It’s 9h31m. That’s how long it is between 6:49 pm to 4:20 the next morning.
Alternative less hacky but much slower method: adding 11 minutes makes 7 pm, adding five hours makes midnight, adding four more hours makes 4 am, adding twenty more minutes makes 4:20 am. So we’ve got 11m + 5h + 4h + 20m = 9h31m. Sometimes looking at an analog watch helps with this alternative method.
By the way, when subtracting, be clear about what kind of span you’re looking for. If you have volume one of a book series all the way up to volume ten, that’s not 10−1 = 9 books. It’s ten books. But the distance between a city one mile away to a city ten miles away in the exact same linear direction, that is nine miles. And with times and dates, sometimes you want one, sometimes the other. We were there on January 1st and we were there on January 10th and all the days in between, but then we weren’t there anymore. That’s ten days, not nine. Tricky stuff.
Speaking of dates, the lesson two of this hack for when the timestamps are more than 24h apart, that would involve equivalents to the “subtract 40” rule. “Day 23, 6:40 to day 49, 7:45” is easy to figure out (it’s 26d1h5m) but “Day 23, 7:45 to day 49, 6:40” not so much. If that’s formatted as 230745 to 490640, subtract 10000 and add 2400, (a.k.a. subtracting 7600 in one go if that’s easier. Which it’s usually not). So we’ve got 230745 to 483040. Then if the minutes hack is needed, which it is in this case (and we can tell it is since 40 is less than 45), subtract 40 as earlier, leaving us with 230745 to 483000, and then subtract and put the d h m markers back in and it’s 25d22h55m in this example.
Again, programmers, do not do this. This is only for, I dunno, scouts or something.
I actually do use some of these hacks, like in the laundry room I like mentally checking that the next date is seven days later which is easy if day one is 7 and day two is 14, but not so much if day one is 26 and day two is the next month’s 2. Here, I add the number of days in the first month (31, say) to that second date so I can compare 26 to 33 and be happy with that being seven days ahead. This is in addition to the calendar GUI in the laundry room wall app, it’s just me being too neurotic and scared of misclicks to entirely trust that but too lazy to whip out an actual calendar.
Similarly the “24h clock but crossing midnight instead of noon” hacks is great for figuring out how long I slept. And that subtract 40 is probably the useleast and controversiallest one of them all but it can be great for tricky times when my brain is on the fritz. “4:49 to 6:07 is… Uh…” While thinking of it as 4:49 to “5:67” is way easier, it’s 1h18m. (A normal person would’ve just added 11 + 7 and then remembered to “carry the one” in some way that makes it one hour not two and okay good for you! I’m glad there are many methods.)