T-Mobile's Data Stash goes into effect at 12:01am
Each month, your leftover data will be computed to the nearest MB and you will have 12 months to use it. If you're very frugal with your data (using Wi-Fi whenever it is available is a big step), you could end up with enough leftover data to watch all of the episodes of Grey's Anatomy on Netflix, while using T-Mobile's 4G LTE network.
Is this the best Un-carrier move made by T-Mobile to date? After all, there is something to be said about getting an opportunity to use the data that you didn't touch during the month. This is akin to dining in an expensive steakhouse, finding yourself unable to finish your rib-eye, but getting invited back to polish off your meal anytime up to a year later.