Starting process of moving lots of similar date handling and navigation code out of Date and Calendar and into the Date API where we can use the same code everywhere. Adding a flexible date argument to the API that will work on any Views date field and take any ISO date argument. Adding an argument default option to set a missing date argument to the current date that will work on any date field. Adding a date back/next navigation attachment that works with the date argument and which can be attached to any view. Next step will be to go in and adapt the Calendar module to share this code instead of creating its own.

Add calendar week calculations to the Date API so we can move them out of Calendar module to be able to use them in any date application.

Fix critical typo in date_get_timezone() that set the timezone to the site timezone even if it isn't right.

Fixes needed to display the timezone when that format is selected, and to pull the timezone out of format strings.

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.