The mini calendar does not correctly display for the months in which Daylight Savings changes.

For example, for the month of November, the calendar will start at Nov 1, 00:00:00 local time and end Nov 30, 22:59:59 local time.

This is caused by the display->tend calculation which starts with the display->tstart time and adds the number of seconds for the number of calendar days in the month. However, when DST changes in a month, the number of seconds in that month will be off by an hour either direction.

The mini calendar instance of the miscalculation points to a larger problem in moodle for calculating the end of a month. For example, I see this type of logic in calendar\export_execute.php among others.

Succinctly, if the end of a day, month, year is calculated based on the standard length of a day (usually using DAYSECS), then end of the period will be incorrect for any period including a Daylight Savings change.