moment.js being mutable is a pain. What're people using instead of moment?

Being insistent with integration partners to accept/send a unix timestamp, so I don't ever have to mess around with conversions.

It would be nice if the whole world traded in UNIX timestamps but that's not massively relevant here.

I use day.js

Haven't used it personally but js-joda is also reported to work

date-fns https://date-fns.org

If you want performance use date-fns If you want timezone conversion luxon I used luxon because it supports timezone straight away. date-fns requires additional library to handle timezone.

I'm looking at switching to date-fns

I've decided I should set all custom field dates with the same timezone as the company configuration.

what is the company configuration changes?

You should build a small module to encapsulate the date conversion logic, and the module should always check the company configuration. This way, you’ll have a future-proof, maintainable solution for managing dates.