It's quite annoying to deal with different library dependencies that all picked a different userspace library for something that should be standardized. For example, you might ship a date picker component using dayjs, but I already adopted Luxon in my application, so now I need to write some janky adapter or end up using both.

I would love to not have to import dayjs or another userspace library, so that i can pass datetime-with-timezone to other libraries and instead use an IETF standard way of handling time that's backed by performant native runtime code.

In my experience, userspace timestring parsing and timezone conversion can also be a surprising CPU bottleneck. At Notion, we got a 15% speedup on datetime queries by patching various inefficiencies in the library we selected. The runtime can be more efficient as well as saving page weight on tzdata and locale stuff.