Your billing cycle is different from when your bill is actually due for payment.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
As mentioned, your usage cycle date is different than your billing cycle date. Example...my billing due date is on the 4th of every month, but the end of my usage cycle is the 9th (new minutes and text count starts on the 10th). If you look in My Account and click on Usage you should be able to see what your cycle date is.
Ur minutes start over 6 days after the bill is due. New bill comes out to be seen 4-5 days after new minutes. A good day to make changes to avoid prorations is the day u get ur mins or future date to that day. It takes a day or so for my Verizon to realize usage for new min.
Hope I helped.