You should store dates with timezone or you can't make calculations like how old is this post. You could base it off system time, but then the calculation have to take into account summer/winter time.
I think you misread the post you responded to.
It is discussing the benefit of Date Vs. DateTime data types. In Postgres, by design, a date has a 1 day resolution (i.e. no concept of hours/minutes, within that date). This is hugely useful for scenarios where you intentionally want and only want a 1 day resolution. It not being timezone adjustable is a feature, not a bug.
Storing a DateTime with 12:00 and then an offset of e.g. 0, is a needlessly complicated work-around when a data type that doesn't even have the concept of hours or timezones exists.
For example, in a lot of countries, Labor Day is on May 1, Christmas on Dec 25. These do not happen at a certain time, and there is no timezone to be associated with these events.
If I'm requesting some user's birthday, I'm going to store it as a date. I don't have a time component. I don't know the time zone. In some cases, dates are just dates ...
For example: If a user schedules an event to happen at 15:35 on July 29th 2021 IST, you cannot know with certainty what the equivalent UTC value is. You know what it would be assuming the relationship between IST and UTC stays the same between now and next year. If India suddenly decides to implement some sort of DST, or make any other changes to the definition to IST, the UTC value you stored is now wrong.
Timezones are political, but they're also how people organise their lives. The user's local timezone is context that you can't just discard.
It is not wrong. When timezones change, the database that contains that info is updated - but it still retains information about historical rules, precisely so that earlier dates can still be converted reliably.
On the other hand, if you use local times, then you are not able to distinguish between pre- and post-clock change, when the same time repeats twice.
But dates should never be stored with a timezone at all, except in extremely rare cases where a day for some reason specific to a timezone. But even then, it's much more likely that you are trying to specify a time range that happens to start and end on a certain day in a certain place.
Dates, almost by definition, are independent of timezone. January 1st, 2020 began at several different times in different timezones. It should be represented as a date stamp: 2020-01-01 without any timezone.
If you really are trying to represent the day that starts at 12:00 AM PST on January 1, 2020 and ends at 12:00 PM PST the same day, then you probably just want two timestamps representing those instants and not a date at all. But again, that would be quite rare. In most cases you would just want to store 2020-01-01.