That doesn't fix the problem of "my quota reset (or reporting job) gets shifted by 1 hr when daylight saving time changes".
All of yours might well be, but you can't generalize that assumption to every domain and scenario.
How are you going to do this? Clearly cron (with timezone set to UTC) doesn't work for this so you'll need another system. In that case why not have that multi-time zone scheduling system run your script?
But also, if the user can change the timezone themselves, now you have a condition where the user could keep their quota at bay for a very long time (if not forever) if they keep changing their timezone accordingly.
Instead of tying your quotas to calendar days in some specific time zone, tie them to a rolling 24 hour usage period. Even better: use a rolling hour.
How hard is to convert the UTC time to user's local time and perform quota reset?
I mean you should be doing this anyway. Even if you store everything in PST or EST or CST, you never know when you'll get customers from another part of the world and then you'll have to do this anyway. So why not do this already?
Probably roughly as hard as writing a timezone-aware scheduler that considers all edge cases around daylight savings time, i.e. probably possible but I'd try fairly hard to see if I can get around it.
One way of getting around it is to run your batch jobs in your "primary business timezone". Sure, you might outgrow that concept eventually, but many companies never do, and in that sense "running on UTC" seems similarly aspirational in spirit (albeit at a much smaller scale) to starting with high scalability.
1. allowing users to change their timezone and tying it to when their quota reset sounds like a great way to add more edge cases and complexity. For instance, does messing with your time zone mean you can get your quota reset multiple times a day?
2. Not everyone operates some sort of global b2c SaaS that's timezone agnostic. For many enterprise backoffice tasks "6am HQ time" is far more reasonable than "6am/7am, dunno depends on whether daylight saving time is on or not". In this case having a server set to the HQ's timezone makes far more sense than doing UTC and trying to work around daylight saving issues.