Question

I’ve run into a strange timewarp while doing some math with time, and it has left me stumped. Or, well, I’ve found the reason (or atleast a plausible one), but don’t quite know what to do about it, or if there is indeed anything that can be done.

The issue in plain words is, that when adding time in larger units than 1 week (it’s multipliers excluded) it seems to be impossible to be exact. And by exact I mean, that when I add 1 years worth of seconds to NOW, I end up 1 year and some hours from NOW.

I can understand that adding 1 (or more) months will do this, as a months length varies, but a year should be more or less the same, shouldn’t it?

Now I know you’ll want to know how I do this, so here follows (pseudoish) code:

class My_DateInterval {
    public $length; // Interval length in seconds

    public function __construct($interval) {
        $this->length = 0;

        preg_match(
            '/P(((?<years>([0-9]{1,}))Y)|((?<months>([0-9]{1,}))M)|((?<weeks>([0-9]{1,}))W)|((?<days>([0-9]{1,}))D)){0,}(T((?<hours>([0-9]{1,2})){1}H){0,1}((?<minutes>([0-9]{1,2}){1})M){0,1}((?<seconds>([0-9]{1,2}){1})S){0,1}){0,1}/', 
            $interval, $timeparts
        );

        if (is_numeric($timeparts['years'])) $this->length += intval($timeparts['years']) * 31556926; // 1 year in seconds
        if (is_numeric($timeparts['months'])) $this->length += intval($timeparts['months']) * 2629743.83; // 1 month in seconds
        if (is_numeric($timeparts['weeks'])) $this->length += intval($timeparts['weeks']) * 604800; // 1 week in seconds
        if (is_numeric($timeparts['days'])) $this->length += intval($timeparts['days']) * 86400; // 1 day in seconds
        if (is_numeric($timeparts['hours'])) $this->length += intval($timeparts['hours']) * 3600; // 1 hour in seconds
        if (is_numeric($timeparts['minutes'])) $this->length += intval($timeparts['minutes']) * 60; // 1 minute in seconds
        if (is_numeric($timeparts['seconds'])) $this->length += intval($timeparts['seconds']);

        $this->length = round($this->length);
    }
}

class My_DateTime extends DateTime {
    public function __construct($time, $tz = null) {
        parent::__contruct($time, $tz);
    }

    public function add(My_DateInterval $i) {
        $this->modify($i->length . ' seconds');
    }
}

$d = new My_DateTime();
$i = new My_DateInterval('P1Y');
$d->add($i);

Doing some debug printouts of the interval length and before/after values show that it’s all good, in the sense that “it works as expected and all checks out”, but the issue stated above still stands: there is an anomaly in the exactness of it all which I’d very much would like to get right, if possible.

In so many words: How to do exact mathematics with time units greater than 1 week. (I.e. 1 month / 1 year).

PS. For those wondering why I’m using My_* classes is because PHP 5.3 just isn’t widespread enough, but I’m trying to keep things in a way that migrating to built-in utility classes will be as smooth as possible.

Answer 1

A year is 365.25 days, roughly. Hence we have leap years.

Months have variable lengths.

Hence the semantics of what adding a year and adding a month probably don’t correspond to adding a fixed number of seconds.

For example adding a month to 14th Feb 2007 would probably be expected to yield 14th March 2007 and to 14th Feb 2008 would ive 14th March 2008, adding 28 or 29 days respectively.

This stuff gets gnarly, especially when we add in different calendars, much of the world doesn’t even have a February! Then add “Seven Working Days” – you need to take a public holiday calendar into account.

Are there no libraries you can find for this?