Just a few weeks after T-Mobile US started letting customers keep unused data for up to a year, AT&T today announced a similar program, but with stricter limits. Eligible AT&T customers will keep unused data for one month.

"Beginning January 25, all new and current AT&T Mobile Share Value customers will automatically receive the Rollover Data feature at no additional cost," AT&T said today. "Now, all Mobile Share Value customers will have their unused, shareable plan data in a given month roll over to be used within the next month. The best part is that it is simple and easy to track, and Rollover Data benefits the whole family or business—across all smartphones and devices on the same Mobile Share Value plan."

More than 50 million customers will receive the rollover data, AT&T said. In all, AT&T has 118.7 million wireless subscribers and connections, 75.1 million of which are postpaid. The rest are classified as prepaid, reseller, and connected devices. Prepaid customers aren't eligible for rollover data, nor are people grandfathered into older data plans. Some of the older plans have unlimited data, however.

T-Mobile says it plans to extend rollover data to prepaid customers, but it hasn't done so yet.

AT&T was apparently planning the rollover program even before T-Mobile announced its own initiative. "According to records from the US Patent and Trademark Office, [AT&T] filed on Nov. 25 to trademark 'Mobile Share Rollover,' 'Rollover Data' and 'Family Rollover,'" The Wall Street Journal wrote last month.

AT&T's rollover program does have at least one benefit over T-Mobile's similar "Data Stash." While T-Mobile's rollover data is only available to Simple Choice smartphone customers who pay for at least 3GB a month (or tablet customers who buy at least 1GB a month), AT&T allows customers with any size Mobile Share Value plan to use data rollover. T-Mobile does give all eligible customers a free 10GB in their "Data Stash" to use over a 12-month period.

Rollover is also more important for AT&T customers. When T-Mobile customers go over data limits, they can continue to use the Internet at lower speeds at no additional cost. AT&T automatically charges customers extra when they exceed data caps. Hopefully, that will happen a bit less often after the data rollover program takes effect. And with any luck, these moves by T-Mobile and AT&T will pressure Verizon Wireless and Sprint to give their customers the same perk.