# [tz] Direct implementation of mktime_z

Tue Sep 27 04:17:36 UTC 2022

```On Sat, Sep 24, 2022 at 1:17 AM Ted Phelps via tz <tz at iana.org> wrote:

> On 2022-09-24, Tony Finch wrote:
> > You might also like https://dotat.at/@/2008-09-10-counting-the-days.html
> > (That does things the hard way, rather than relying on tm_yday)
> > and https://dotat.at/@/2008-09-15-the-date-of-the-count.html
>
> Wow, that's some clever math you've done there; far nicer than the
> clumsy approach I'd used to canonicalize the year, month and day.  It's
> a little slower than my clumsy approach in the case where the year,
> month and day are already in canonical form, but can be much faster when
> they're not.  I'll incorporate those changes into my git project in the
> coming days, thank you!
>

static inline int64_t daynum(int64_t year, int mon, int day)
{
// Rotate the start of the year to March so that the troublesome
// leap day is last.  Also, make March month number 4 to simplify
// the calculation below.
if (mon > 2) {
mon += 1;
} else {
mon += 13;
year -= 1;
}

// Compute the day number since the start of year 1.  This clever
// expression is thanks to Tony Finch; see his blog post for a
// detailed explanation of what's going on here:
//     https://dotat.at/@/2008-09-10-counting-the-days.html
return year * 1461 / 4 - year / 100 + year / 400 + mon * 153 / 5 + day
- 428;
}

Assuming we want to model the proleptic Gregorian calendar, it would be
nice if the output was linear over all inputs.  At the moment, for
example, daynum(0,
2, 29) and daynum(0, 3, 1) both return -305.
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