Also valid. I just use ISO8601 and bite the bullet because storage is cheap.
This runs on python and works with several backends including SQLite. https://requests-cache.readthedocs.io/en/stable/
SQLite is used in some HPC work.
ISO-8601 datetime strings can easily wreck your L1 cache. Instead of filling an eighth of the cache with a 64-bit value, you wind up filling almost half with a 27-character-long string.
What format works well for hardware caching?
As small as you can tolerate tbh. The game is played by keeping data as small as possible and ensuring that related data has good locality in memory.
The gap between how fast your CPU works and how fast data can be retrieved from RAM is enormous.