It's one of the stereotypical spy code phrases used in bad and/or spoof movies. I've seen it credited to Top Secret, but not having seen that movie, I can't vouch for the assertion.

I've also seen it as "the rooster crows at midnight", or "the eagle flies at noon". Alas, my Google-fu is not up to finding a definitive source.

Origin and meaning of "The eagle flies at midnight"
When the anime starts and until episode 8, they attached a cell-phone to the phone-microwave and sent a message to that specific phone to trigger sending D-Mails. But in episode 8 


   Ruka sent a D-Mail to her mother pager


and then in episode 9


   Faris sent a D-Mail to (probably) herself


So I'm wondering if there is a plot whole somewhere in the story, or did they figure out a way to send D-Mails from any phone to any phone? 

As a side question previously it was mentioned that how they can set D-Mail's time of Arrival:


   each second on Phone-Microwave meant an hour going back, so I'm wondering how did they manage to set microwave time to 87600 (10 years)?

