Another consequence of this is that JS Date objects don't understand the concept of timezones, and only allows you to output dates in either the client timezone or UTC. This makes it extremely hard to, for example, display the arrival time of a flight in the destination timezone.

Intl.DateTimeFormat[1] is a godsend for this.

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...