Publish datepicker time zone is server time instead of user time

[iandunn]

What problem does this address?

When scheduling a post, or editing its publish date, the datepicker shows the date/time in the time zone that WordPress is configured with.

If that doesn't match the user's timezone, then they have to manually convert that to their local time, which is tedious and error-prone. Many users will probably not even realize that there's a difference, and end up setting a different time than what they expect.

Related: #15673 / #23400, #25256

What is your proposed solution?

The datepicker could display the time in the user's time zone, so that the interface is intuitive and easy to use. When the post is saved, that value could be converted to the server time zone (or stored in UTC, or whatever the internals require).