This is not only a bold claim, but utterly wrong. Every single datetime you’re storing should absolutely contain a timezone, preferably UTC.
I‘ve been bitten by dates lacking timezones in various situations during my career and it has always been a pain in the ass to debug and resolve all those random problems they‘re causing.
My understanding is that you don't even want to store them as UTC and always store them in the timezone of the user/system that generated the data since you lose some information in the translation from local timezone to UTC - given that the timezone database changes, you may actually want to know that the record created in Fiji before 2022-10-28[1] was stored in DST so that it can be properly converted to the new Fiji time.
This! People mistake the offset for the timezone, which is incorrect. The offset isn't very useful other than a guestimate of the originating timezone.
Ideally store both, UTC and original local time, or at least the original TZ information. Time and timezone are a huge mess and one of the worst things to try deal with in programming.
In the airline industry they sling huge numbers of dates and times around, and they never include the timezone. What do they include? The airport code. Boarding, departure, arrival, and all other times are always local to the airport. In practice, that means that every piece of code that wants to handle datetimes has to also have access to a way to lookup the timezone for the provided airport code. Have a time at LAX? look up LAX, find the timezone, and compute the offset.
In practice, almost nothing does TZ conversions or datetime math. In effect, airline timestamps are a tuple <datetime, airport_code>, and nothing much messes with that.
Oh, it's not great at all. The only upside to it is that passengers looking at schedules and boarding passes don't have to do any timezone conversions for long flights. The departure time is always the time at the origin, and the arrival time is always the time at the destination.
I am that donkey banging it's head on the same post; when I see dates stored without TZ, I assume it's UTC and have done so for decades. It's turned out to almost never be true.
Yes! I have spent so much time detangling timestamps in data which used non-UTC times and non-standard formats. Like 06-10-01 WTF is that? Use UTC and then let the reader translate to local time on display.
I‘ve been bitten by dates lacking timezones in various situations during my career and it has always been a pain in the ass to debug and resolve all those random problems they‘re causing.
Do. Save. Dates. Containing. Timezones.