Angel of the Dire Hour

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2014-11-07
  Official Ruling
  If you cast Angel of the Dire Hour outside of combat, its triggered ability will trigger but have no effect.
• WWizards of the Coast2014-11-07
  Official Ruling
  If a spell or ability instructs you to put Angel of the Dire Hour onto the battlefield from your hand, that means you won’t have cast it, so its triggered ability won’t trigger.
• WWizards of the Coast2014-11-07
  Official Ruling
  Creatures that were declared as attackers or entered the battlefield attacking continue to be attacking creatures until the combat phase ends. It’s possible to exile all attacking creatures after they’ve dealt combat damage if you cast Angel of the Dire Hour during the combat damage step or end of combat step.

Rulings via Scryfall