Wireless carriers in the US are on a roll -- or more precisely, a rollover.

In December, T-Mobile unveiled a program called Data Stash that lets customers carry unused mobile data from one month into the following months.

And now AT&T has announced that it will offer rollover data to its own Mobile Share Value customers, the newcomers as well as the existing ones.

AT&T's program, announced Wednesday, allows for any unused data a customer has paid for during the month to roll over to the next, at no additional charge. For instance, if a family pays for four lines and 15GB of monthly data and then in a given month uses 12GB, the 3GB of unused data will be added to the next month's services -- in effect, giving that family 18GB of data for the price of 15GB. All of the data is shareable, meaning all lines can use the additional storage.

But the deal, which goes into effect January 25, has its limits. According to AT&T, "unused data expires after one billing period or with any plan change." In other words, if the unused data is still not used during the rollover month, it goes away with the next billing period.

Meanwhile, T-Mobile's rollover program also goes into effect this month. The Data Stash offering allows any customer with plans featuring 3GB of monthly data or more to roll over unused data to the next month. That rolled-over data gets stored for the following 12 months. In addition, new and existing customers get an initial 10GB of free data for the "stash" to kick things off, and that bonus data lasts through 2015.

"If you buy data, it's yours," T-Mobile CEO John Legere said on a webcast after the announcement last month.

With the dueling rollover programs, AT&T and T-Mobile are stoking an already hot trend among carriers looking for new ways to entice subscribers. That's been good news for deal-savvy consumers, but not so good for the carriers' finances.

AT&T's new offer should "increase pressure on Verizon to offer a similar deal," Jackdaw Research chief analyst Jan Dawson wrote in a statement Wednesday morning, "though Verizon has resisted recent moves to a greater extent than competitors, preferring to target discounts at individual users it considers at risk rather than sweeping discounts or widely-available offers."

The rollover concept has deep roots at AT&T, which in an earlier incarnation let customers take unused calling minutes from one month and add them atop their next month's plan to limit the chance of incurring overage charges. AT&T eventually abandoned the feature when the industry turned to unlimited talk time as data became the new expensive component in wireless plans.

In fact, AT&T owns the rights to use the term "rollover."

On Wednesday, Legere took to Twitter to taunt AT&T, saying that it's "only day 7 of 2015 and my predictions are coming true." The outspoken CEO previously said that T-Mobile was breaking ground by offering its Data Stash and that it expected competitors to follow its lead.

However, an AT&T spokesman said in an emailed statement that his company "always planned to announce this at the Consumer Electronics Show to start the year with a great new benefit for our customers."

That was corroborated by analyst Dawson, who also noted some key differences between the offerings from the two carriers:

AT&T's version differs significantly from T-Mobile's version. It's less generous, with a single month of rollover rather than twelve months, but that should make it both easier for customers to keep track of and more manageable from a network load perspective. T-Mobile's plan risks creating a situation similar to airline miles, where customers have a hard time keeping track of which miles (or Gigabytes of data) expire when.