Due date change and reduction in signal

Hi,

I have 2 issues:

1. I recently noticed that my billing date has changed from 13th of each month to the 3rd of each month.  I would like to know why this is so because i prefer the original due date? 
2. For approximately 5 or 6 months now, I have noticed that my signal has been extremely low or none-existent in my workplace.  AT&T use to work fine and have a stronger signal up until like i said previously 5 or 6 months ago.  Has something changed?