Post scheduling date is broken by entering a non-existant date.

[caesarsgrunt]

For example, it is possible to enter the date as 31st Nov, which doesn't exist. The date is then reset to 1st Jan 1970.

Either it should be impossible to select dates which don't exist, or else the example above should be automatically changed to 1st Dec.

[azaozz]

Dupe of #10309 fixed in [12318].