Incorrect session time calculation during DST transition (clock shift)

[yonatankostov]

Summary

There is an issue with session time calculation in mod/attendance/locallib.php when daylight saving time (DST) changes occur. The current implementation calculates times starting from midnight, which causes incorrect session durations when the clock shifts (e.g., at 02:00).

Code Reference
$sesstarttime = $formdata->sestime['starthour'] * HOURSECS + $formdata->sestime['startminute'] * MINSECS;
$sesendtime = $formdata->sestime['endhour'] * HOURSECS + $formdata->sestime['endminute'] * MINSECS;
$sessiondate = $formdata->sessiondate + $sesstarttime;
$duration = $sesendtime - $sesstarttime;

Problem

The calculation assumes a continuous timeline from midnight (00:00). However, during DST transitions (e.g., when clocks move forward at 02:00), one hour is skipped

[danmarsden]

I vaguely recall some changes to some of that handling - can you please always make sure you include information on the moodle version you are using and the specific version of attendance you have installed?

I would definitely like to see some improvements to the way timezones are handled on sessions - we've also have previous requests for being able to specify a specific timezone when creating a session (like facetoface does.)

If you have internal development capabilities, feel free to help further by submitting a PR.

thanks,

[NeillM]

It looks as though that code is still there in the latest version, I guess another edge case where it might cause issues is in non UCT timezones on a day when a leap second is added (although I imagine that edge case is much less likely to be noticed, it is also much less frequent)

[wiktorwandachowicz]

Hello, I was trying to overcome such problem in #564 several years ago. But the solution was accepted then dropped in #683, and for a good reason! Because only date() was proposed at that time.

In our university we have a small patch for single sessions - relying on combination of userdate() and make_timestamp() calls. Code for adding multiple sessions seems okay, though.

This approach mirrors existing solution for multiple sessions, to be used in similar way for single sessions too:

// File: locallib.php

function attendance_construct_sessions_data_for_add($formdata, mod_attendance_structure $att) {
    // ...
    if (isset($formdata->addmultiply)) {
        // ...
                    $sess = new stdClass();
                    $sess->sessdate = make_timestamp(
                        $dinfo['year'],
                        $dinfo['mon'],
                        $dinfo['mday'],
                        $formdata->sestime['starthour'],
                        $formdata->sestime['startminute']
                    );
        // ...
    } else {
        $sess = new stdClass();
/* Current code */
//      $sess->sessdate = $sessiondate;
/* Improved code */
        $sess->sessdate = make_timestamp(
            userdate($formdata->sessiondate, '%Y'),
            userdate($formdata->sessiondate, '%m'),
            userdate($formdata->sessiondate, '%d'),
            $formdata->sestime['starthour'],
            $formdata->sestime['startminute']
        );

In the end there is a need only for Unix timestamp. Just extract proper year, month and day using userdate() - which should take into account Moodle user preferences and return what user was intending. Then use make_timestamp() which should correctly handle transformation to timestamp, also utilizing DST information.

@yonatankostov
It would be great if you could try this change and confirm whether it solves the problem.