Steps to reproduce:
1) Create "Local Live" staged site and some test content on it
2) Click "Publish to Live" on the top of the page
3) Click "Switch to Advanced Publication" in the dialog
4) Click "Scheduled" under "Date"
5) Click "End by" and set a time which is before "Start date"
6) Click "Add event"
Result: "Portlet is temporarily unavailable." is being displayed and adding an event fails.
There should at least be a more informative error message (like when editing web content and putting expiration date before display date) or user should be informed and error condition prevented if going into settings like these.