Years ago, I found an easy way to convert hours and to add 9, 8, 7, and 6 to any number. Some people seem to struggle to convert hours, and adding units manually can be hard, but I found an easy way for these things. I'm not sure if this is public knowledge, I had to figure this out on my own, so I'm putting it here. This method has helped me a good amount of times, and I always use the time converting method.
First, for hours: You just add or remove 12 to convert them. So if I was at 11:00 PM I could add twelve and it would be 23:00. To make things easier, you can just add 2 then add 10. My thought process always goes like this: 11:00 -> 12:00 -> 13:00 -> 23:00 or better 18:00 -> 07:00 (automatically reduced 10 and 1) -> 06:00
To add 9, 8, 7, or 6 units, remove X units according to the number and add a dozen. In summing, for 9, you remove 1 unit, for 8, 2 units, and so on and you add a dozen always. This came naturally after I learned that 9 in the multiplication table increases dozens but decreases units. So if I were to add 8 to 23 it would go like this: first remove 2 so 21, then add 10 which becomes 31. Much easier than calculating by hand.
And for subtraction, just do the inverse, add X unit and remove a dozen.
This makes for very easy multiplication table making. I just add a dozen and remove 2 for 8 for example. 8 16 24 32 40 48 56 64 72 80
Numbers smaller than 5 this method doesn't really work, it's easier to sum by hand, it's also not worth doing this method for numbers that end with 0, it's easier to just add it by hand (if you were to do the method anyways, it would go like this, supposing i'm adding 9: 30 -> 40 (add dozen) -> 39 (remove 1 because it's 9)).