The zero padding is in fact part of the format.
> ISO 8601 - Verbose
It's an extra few characters but I can also tell what date it is and what time of day it is by looking at it.
Also you can remove the separators to make it only 4 characters longer.
On the other hand if you really want it to be compact, AlphaDec doesn't do the job. To go from year down to second you need 8 characters, and then 3 more for milliseconds. Base32 would need 5 and then 2 more.
There's probably a good middle ground here but I think AlphaDec sacrifices too much. I'd much rather have something that encodes month, day, and hour into one character each so even if I didn't memorize which is which I can at least truncate by them instead of periods, arcs, and bars.
I think the pure 4-character version has some good appeal, but both truncating it and extending it for better precision lose most of that appeal.
> ISO 8601 - Not great for filenames or embeddings
I guess, but using a different character from colon is a very simple tweak you can make.