Interestingly, sometimes the system is used for future dates. For instance, my drivers license expires in September of H.31, which I guess will never happen now.

Does this imply that the driver's license becomes invalid and must be replaced with one in the new date system?

Real question

Or -- perhaps more intriguingly -- given that the date in question will never occur, is it valid until the end of time?

Philosophically it's a rather fun question, but practically they'll probably just honor it until the corresponding date of the new era.

It will stay valid. Consider having have to replace millions and millions right after the swap, it will be chaos. The system is simply stupid, not even citizens want it.

Canada isn't much better re: feet, miles. Things are officially metric, but because of proximity to the US and inertia, half of everything is still standard. Buying construction supplies and consumables? All in US units. People talk about square footage of condos, buy carpet in square feet, all fasteners are in US units. But gasoline is sold by the liter. People know how tall they are in feet and inches, not cm. Everything is mixed up together.

Interesting. UK is also weird: food is weighed in metric units, people in imperial; Drinks bought in supermarkets are measured in metric, pubs use imperial (by law in both cases, IIRC); Distances on signs can be meters or miles, both initialised as “m” (context being the only clue); fuel is sold by the litre, fuel efficiency is miles per gallon.

Beer and cider is sold in pubs in imperial pints (bigger than US pints), but wines and spirits are sold in milliliters.

See https://www.gov.uk/weights-measures-and-packaging-the-law

Good point, I’d forgotten wine and beer were different. I wonder what further examples of mixed up units the UK has?

Well there is a tendency to still use Fahrenheit during summer like "it's in the 80s", and "it's in the 90s" this month, but most use Celcius for winters when it gets below freezing. There must be a grey area in the 50s and 60s F. Both approaches often seen in headlines. :)

I'm a physicist and I still feel compelled to die on the following hill, re: Fahrenheit---it just maps better to human perception:

30s: Wear a heavy coat

40s: Wear a light coat

50s: Wear a jacket

60s: Wear a hoodie

70s: Wear a t-shirt

80s: Wear shorts

Works in C too:

-ve Don't go outside

0s Wear a coat

10s Wear a jacket

20s Wear a t-shirt

30s Don't go outside

-40s too cold

-30s cold

-20s February

-10s beanie optional

-0s can't store non-alcoholic drinks on the balcony

+0s wear waterproof boots

+10s no need to wear a jacket

+20s don't wear more than a t-shirt

+30s can't sleep since my apt. has no AC

Fixed that for northeners you insensitive clod.

What a brilliant comment! In fact, I'd even say it's over 9E+3 photons/s/mm^2/mrad^2/(0.001*Δλ)

c is just easy because 0 is the freezing point.

Miles per Imperial gallon.

don't some parts of Europe also use decilitres for beverage packaging? I've seen bottles that are like "75dl", and it took me a few minutes to clue in.

That would be centiliters, for example, wine is most commonly sold in 75cl bottles. Millilitres is also a common unit for smaller quantities in e.g. cosmetics.

Right! I actually meant centiliters - it's been a few years. But a commenter below mentions deciliters are a thing as well. What a crazy world :)

At least in some parts of central Europe it is common to buy beverages per decilitre and meat by dekagrams.

750 Liters would be a very large beverage! A typical keg of beer is only around 59 liters.

1dL = 0.1L

My bad, thought the d was for deca, not deci.

I'm in the US and although I spent some time in Canada I am for the most part not used to seeing the metric system used day to day. My metric system knowledge basically is what I remember from elementary and middle school.

Any movement to slowly unify the spreads?

but to further confuse things, your height on the driver's license is in cm, not ft-in. and while some imperial units are widely used, I've never heard anyone using yards or gallons, and very rarely miles. I find it rather adorable.

concrete pours are still priced by the cubic yard (such as if doing a radio tower foundation or any commercial project).

a lot of construction related things that are priced by the cubic volume are still in cubic yards.

Yeah

I suppose they like using lbs and sqft as $/lb is smaller and sqft is a bigger number than sqm (and of course the inertia which I prefer calling "being lazy")

It is definitely ridiculous

It's sad this date system is there for no reason today. Systems never like it and neither do people. I rarely use it and it's mostly bank related but every time I do, I look it up on Google to make sure I remember the number right. This needs to be dropped.