If you are an AT&T mobile subscriber on a non-legacy shared data plan and on their current mobile value shared data plan, after the 25th of this month you will carry over any unused data (rounded up on the whole Mega Byte) to the next month.
This is a trend that has been set by TMO. Unlike TMO, who will allow you to stockpile unused data for a year, AT&’s version of data rol-over lasts one month. So any unused data that is +1 month old goes in the bit bucket.
Rollover DataSM is only available with Mobile Share Value® plans. Unused data from the monthly plan allowance rounds up to the nearest MB and carries over for one billing period. Unused Rollover Data automatically expires after one billing period, with any plan change, or when a group or account is terminated. Unused Additional data resulting from data overage is not included. Rollover Data is always consumed last, after your other data allowances. Unused Rollover Data is not redeemable for cash or credit and is not transferable including to other Mobile Share Value groups on your account. Mobile Share and Mobile Share Data-only plans are excluded.
Per this site:
It’s a token step forward, but better than nothing. If you are an AT&T customer, you might want to check it out if you are on a shared data plan.