How do you know what time zone to convert it FROM? Unlike epoch time (Unix time in milliseconds or seconds), "2021-11-21T15:10:35" doesn't automatically mean UTC without a "Z" or "+0" at the end. Without the time offset, that timestamp could be in any of the world's timezones. There's no way to know what moment in time was actually stored/meant.

Even if the user set it in one time zone, if we store it without a proper offset (and preferably a locale), we cannot properly account for the user traveling to another place, daylight savings time, government time adjustments, leap time, etc. These complexities compound off each other and make proper scheduling and debugging time issues really, REALLY hard later on.

In the twenty years or so that I've been doing web dev, datetime is probably the single biggest gotcha I've seen developers (including myself) stumble into. Part of this is because the standard JS datetime handling is atrocious compared to any other language I've worked in. It doesn't really natively handle time zones well, and well carelessly cast them back and forth if you're not careful.

Moment and Luxon can help with this, lighter libs like date-fns can too with the right extensions, but out of the box JS is going to fuck it up. I promise. Another choice is to just use milliseconds to store everything, and convert that to the user's timezone only at the time of display. Every timestamp is stored as an epoch, and every user has their time zone (not just offset, because of daylight savings) recorded with their profile. Doing those two simple things can avoid most of the landmines.