Fix for critical flaw in the new timezone list handling that was creating an invalid array. This should fix it.

Add missing strtotime validation and format help text to textfield date and clarify that it's really a custom input format (which uses strtotime if the custom input fails).

#255739 Fix backwards validation message when testing that the To date is greater than the From date, patch by parasite.

Prepare for proposed core timezone handling from #11077 by getting rid of deprecated timezones and adding in js timezone name detection.

Rework default timezone handling to force a valid date even when timezones are not set to get rid of numerous installation and operational errors caused by invalid date objects.

Update site timezone offset during cron and user offset during login to make sure that modules that rely on the timezone offset  have current information to process when the offset changes.

Rework the date filter and argument -- simplify some of the code but still keep as much flexibility as possible. You can set up the filter to provide either a date or offset selector (or both), you can set the granularity of the date parts, and any part can be set to all all values, the current time, or a specific value for either a single date or a from/to combination.

#234102 It turns out that we must always force use of the lower level date functions because regular strtotime and mktime can be off by one hour during daylight savings time.

#239900, #249399 Limit available formats for Date Popup to available formats and hide custom format code which won't work right here.

Tweak the recent changes to format interval to simplify processing and make sure they go through the right theme.

#233415 separate ical retrieval and parser so other modules like feedapi can use the parser alone, patch by ekes.