The problem is that there are multiple ways to define a quarter. Jan 1 - Mar 31 is what the (Gregorian) calendar says for the first quarter, but quarters are more commonly used in a fiscal sense and their definitions can vary wildly.
https://en.wikipedia.org/wiki/Fiscal_year

See also bug #15184, but it's been more than a decade so I think this is worth a second thought. Perhaps just going with the simple Gregorian version - which can often be used to derive others.

Although in some of those "other" cases you still need to know the month to make the right adjustment - and if you have that you can calculate the quarter anyway.

For some, yes, but most of them I saw are based on the calendar year so chaining one or two relative amounts should get the job done. For example, US federal is one quarter behind the calendar; its Q1 2018 started Oct 1 2017 and you could get it with "quarter 1 2018 -3 months".

That would work (I'd be using "+ 3 months" and "+ 5 months"); the suggestion in the other bug you cite (#15184) for date('q') on the other hand would be useless for a year starting in June.

It's the difference between "get a timestamp for a given quarter" and "get a quarter for a given timestamp". The former would actually be easier.

Yeah, adding or subtracting months creates a risk of overflowing/underflowing dates resulting in wrong results for a few days of each year. It's okay for the quarter-to-timestamp direction since the quarter would "be" on the 1st of some month, but going in reverse you'd have to deal with February and 31sts.

The issue is complicated enough to prove it needs its own library. As for PHP, I'm still hoping there's some simple solution that can at least be a starting point for most use cases.

Sorry, that other bug didn't show up when I searched for "quarter".

When it comes to calendars, *anything* is a mess. E.g. it isn't even universally clear which year we have: https://en.wikipedia.org/wiki/Hebrew_calendar

But: I have never heard from any other quarters than these: https://en.wikipedia.org/wiki/Calendar_year#Quarters
Even https://en.wikipedia.org/wiki/Fiscal_year mainly sticks to that, some ambiguity is just about the *numbering* of the quarters. So "first|last day of last|this|next quarter" should be fine for everybody.

And there have been even more tricky questions in the past, for which finally a common solution could be agreed upon, e.g.:
http://php.net/manual/en/datetime.formats.relative.php
> Weeks always start on monday. Formerly, sunday would also be considered to start a week.

Bug #15184 has been re-opened in the meantime, so I'm closing this
as duplicate.