This article explains pro rate in and out
What is a prorate? Prorate takes the monthly amount and divides it among the days of the month (essentially makes it a daily rate).
Pro Rate In:
To help illustrate, we will use a fictitious customer, Amy.
NOTE: Gabb has customers on the 1st, 15th, and 23rd billing cycle each month. The following example is a new customer who purchases a phone and is placed on the 23rd billing cycle (23rd of every month).
Amy purchases one Z2 on September 6th (purple box below) and decides to get the Plus Plan. Her total with tax is $133.73 (Z2 device + first month's service + taxes). Her billing date is the 23rd of each month. She receives the phone on September 15th. Amy is concerned because her next bill date is less than 10 days away, and she doesn’t want to pay for another month when she’s hardly had the phone.
The first month of service that Amy paid for on September 6th (purple box) was from 9/23-10/23 (the green line). Her phone was activated on 9/10 (typically a day or two before it ships out). On 9/23, she will be billed for her pro rate in (yellow line) - the days that the phone was activated up until the first bill date. She can expect an $11 bill (24.99 divided by 30, times 13 days of activation). She will be all caught up by her October bill and it will be in the regular amount and cover 10/23-11/23.
This same principle applies to any bill date. If John purchases on September 1st, with a bill date (cycle) of the 15th, his pro rate in would be somewhere around $10 to cover 9/4*-9/15, since his first month (September 15th-October 15th) is already paid for at the time of his initial purchase.
If Susan purchased a phone on December 16th, with a bill date of the 1st, her pro rate in on January 1st would be somewhere around $9 to cover 12/21*-12/31, since her first month (January 1st- February 1st) is already paid for.
If John cancels his service on December 26th, he would have a prorate out (likely refund) of around $17, since there are around 20 days left in the bill cycle, he will never use (12/26-1/15) Remember that his December bill paid for 12/15-1/15.
*This date is just an example of when the phone may have been activated based on the purchase date*