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Date: Sun, 24 Mar 2024 20:50:41 +0000
From: Alexander Weps <>
Cc: Daniel Gutson <>, Markus Wichmann <>
Subject: Re: Broken mktime calculations when crossing DST boundary

See below.


On Sunday, March 24th, 2024 at 21:22, Rich Felker <> wrote:

> On Sun, Mar 24, 2024 at 07:57:39PM +0000, Alexander Weps wrote:
> > > 1. The value of one of the tm_* values it outside of its calendar
> > > range (e.g. tm_min=70). These are reduced prior to any
> > > consideration of timezone mess, producing a nominally valid
> > > calendar date.
> >
> > You are describing the musl behavior, more specifically what I see
> > in mktime & __tm_to_secs.
> > I don't think this is correct behavior.
> > You basically throw away important information and later claim that
> > you don't have it and it's impossible to deduce it.
> This "important information" does not tell us what the caller did to
> get the non-normalized input we received, even if you assume the
> caller just made a single change.
> For example if you see tm_mday=31 in a month with only 30 days, you
> don't know if the caller was trying to move one day forward from the
> last day of the month, or was trying to move one month back from the
> next month.

This is what is called limitations.
I actually investigated this limitation very closely.

And it doesn't influence ordering if you don't handle it at all.

> The reasonable, consistent, least-surprise thing to do is not to try
> to make guesses based on the individual fields and how you think the
> caller might have gotten to them, but instead to normalize completely
> to the ranges before even considering timezone shenanigans.


The reasonable, consistent, least-surprising thing to do is to take a minimal subset of assumptions to make viable ordering.
So time can be predictably incremented and decremented and doesn't go backwards to run into loops like these:

1946-12-01 02:59:17 CET
1946-12-01 01:00:16 CET

1919-11-13 23:59:32 LMT
1919-11-13 23:54:01 LMT

Reasonable is also to behave similarly as other implementations.

> > > You're making test cases which involve both 1 and 2 above, which makes
> > > them more confusing to reason about.
> > >
> > > > But there cannot be a case where you have normalized time add
> > > > something, normalize and create normalized time that is lower and
> > > > vice versa.
> > > >
> > > > If you claim otherwise, provide counter example.
> > >
> > > What I've told you is that, if you compare the broken-down tm element
> > > by element ignoring what zone rule it's under, there will always be
> > > instances where mktime is non order preserving, regardless of what
> > > choices the implementation makes. One way of writing this precisely
> > > is that there will always exist tm1 and tm2 where
> >
> > You made it non order preserving by your choices. You have just
> > shown that the implementation is broken by choices that were made.
> > You can make valid ordering of all struct tm if you consider all of
> > the fields.
> >
> > This is not even relevant to normalization. You can do it on all
> > struct tm just as they are.
> > Normalization should be there to make it easier to do it, not make
> > it impossible to do it.
> No, this happens regardless of the above.

Then provide an example.

> > > This is really not profound. It's just a case of "local times are
> > > lossy in the absence of also taking into account the associated UTC
> > > offset or local time rule in effect".
> > >
> > > I think you've found one real bug where something goes wrong with the
> > > 2011-12-29 corner case, but digging in on other things you think are
> > > wrong, that are just fundamental to how local time works, is
> > > distracting from actually investigating that. Can we try to actually
> > > figure out what's going on there?
> >
> > Sure. But that's not the only bug.
> Well I haven't seen any other credible claims of a bug in this thread.

I am not sure if this is a serious conversation.

I have a struct tm created by mktime. So this is a valid structure that was produced by mktime.
I increment a field (be it second, minute, hour...) in that structure.
I call mktime to give me result of that calculation.
Result of mktime is a s struct tm that has earlier time than the original struct tm.

One of the primary purposes of struct tm and mktime is to make calculations.

Currently the implementation in musl cannot take a date represented in struct tm and iteratively increment it in second intervals to generated all dates up to some date. This basically means, that some calculations are impossible to make. It cannot reliably take a date and add seconds, minutes, hours, days... to that date and get the result.

> Rich

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