I don’t think this is implemented in the standard datetime library, but in principle overriding sub is easily possible and you can define it as you’d wish.
However, I think subtracting a year is a bit ill defined, because it isn’t clear which year you’re subtracting given the leap year issue.
time.now - 10 * time.unit.year
For length, for an average male one meter is about one large step with extended legs (useful for distances), or the distance between e.g. the left side of your torso to the end of the extended right hand (useful for estimating the length of rope or smth).
For weight, it might be useful that 1 liter (that’s 1 dm3 but noone uses that except sometimes in scientific literature) is almost exactly 1 kg, and a typical cup fits 0.25 liter. A shot of alcohol is either 20 or 40 milliliters (0.02 or 0.04 liter) depending on where you are and what you order.
For conversions you just need to remember the base unit (e.g. meter and grams/kilograms) and the decimal prefixes. But you really only need milli (1/1000), centi (1/100) and kilo (1000) in day to day life. Then you simply shift the decimal.