Does AT&T bill you a month in advance for post-paid wireless service?

1) Your cycle date is not necessarily the date you signed up.

2) IIRC, no, you would normally be billed in January on your January cycle date for the part of the month of January to your actual cycle date and the month from January to February, to be paid around a week before your cycle date in February.  In February, you'll receive a bill for the month of service between February and March, to be paid by the due date in March.

(Corrected my info, which is now inline with what MicCheck said.)

As noted, the day you sign up doesn't necessarily correspond with your billing cycle. 

That said, if someone has a billing date of the 15th, you are correct. On January 15, AT&T "prints" a bill for regular monthly service from January 15 to February 14. Since it won't be due until February 7, you are paying after you've used 3 weeks of service, which is what makes it "postpaid."

Any non-recurring charges (international calling, for example) are billed on the next cycle. In other words, a call to Egypt make on January 10 will be on the bill that comes out January 15. 

Another note, while the billing date is the 15th and that's what matters for billing purposes, bills are not actually available online until about 5 days later. If you choose a paper bill, plan on not getting it for about 2 weeks. 

As noted, your bill cycle will not start on the day your service starts. Your monthly bill cycle could start anywhere from 3 days to 3 weeks after start of service, hence your first bill will contain partial charges for those initial days plus the first full month. The bill is not generated until 4-5 days after the start of the bill cycle and is not due until 21 days after the start of the bill cycle. Therefore, you aren’t actually billed in advance, you are billed during your bill cycle. So your example is basically correct except your bill cycle won’t start on 1/15. So if your monthly cycle were to start on 1/18, that first bill would contain partial charges for 1/15-1/17 and full month charges for 1/28-2/17. That bill would be generated on or about 1/22 and would be due on or about 2/10.

As a former AT&T customer and a current Verizon Customer this is the way both carriers bill for service, both for the first bill, and then ongoing bills there after.

First bill:

The first bill usually contains 3 days of prorated service, Plus the entire first month of service. 

Ongoing :

You don't get this bill until you are a full week into the full months service ( so approximately 10 days after you begin service) .  The bill is due a week before the end of your bill cycle.

    In simple terms, you are billed during your bill cycle and it's due during the bill cycle.

When you sign up if you choose to have your bill issued on a certain day, it may change how many days are in your prorated bill.   I find they don't ask anymore. 

Thanks for all your responses.

Now a follow-up question...

If I cancel service a day after a bill gets generated, and I pay that bill in full, am I supposed to get a refund?

For example, the bill date is 1/29 for the billing period 12/27 to 1/26. On this bill, AT&T bills the "monthly charges" in advance for 1/27 to 2/26. If I cancel the service on 1/30, should I be getting a refund for the unused dates 1/31 to 2/26?

No.  At this time the only carrier that Prorates the is Google fi. All other carriers charge you for the full month whether you stay or not. You go even one day into the new bill cycle you are billed for the entire cycle.

Just to clarify, if you are being bill for regular  service from 1/27 to 2/26, your bill date is 1/27. 

I know some here say you’re  billed “during your cycle” because that’s when you see the bill, and normally it doesn’t matter. For your question, though, being precise is critical. If you were to cancel on or after 1/27 (not 1/29), you are still responsible for the bill for the entire month through 2/26. 

Hmm I think comments about mentioning no pro-rate credits are given upon terminating/canceling service, that's not my experience. I've canceled accounts (postpaid) for Wireless plan for both T-Mobile and Verizon, both credit me the pro-rated part, so I only pay for the exact days of service, nothing more or less.

The only situation I can think of no pro-rate case is if your service is "reserved" and not really terminated until the end of month. There was once a T-Mobile representative clearly heard me saying I want to cancel my line, nonetheless still go ahead say my service will remain until the end of month and so does the charge; I jumped in right away and say I want to terminate my line starting TOMORROW, not end of month. I received pro-rated credit at the end. So that's a caveat there you have to make it very clear you want to terminate RIGHT NOW, or tomorrow, not any further. But yes I believe any major carrier will and should give you pro-rated credit. 

As per the TERMS OF SERVICE:

Regardless of the reason or whether your or we terminate your AT&T Services:

• unless required by applicable law*, there is no proration of charges and you are still responsible for the full month’s payment even if your AT&T Services are terminated in the middle of a billing cycle;

*California, Illinois & New York

I am in CA so perhaps that’s why.