How did they start sending messages to other's phone? 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)?
You just send the message from one phone/pager to another one. All you need is the microwave running and a running CRT (Cathode Ray Tube), plus the phone number of the receiver. You configure the microwave, turn it on and wait for the message to be sent.

Earlier in the story, they needed to connect the phone with the microwave, to determine, which message should be affected by time-travel. 


   However, Daru wrote a program, with which you could set the number and various different things, so it was possible to send these D-Mails without the need of connecting the phone to the microwave.


Regarding your time-question: Daru applied many modifications to the microwave, so you could set any running-time. The microwave won't run the whole 24 hours, it just needs to be set to that time.