One video titled “ScienceCasts: Why the world didn't end yesterday” was scheduled to be published on December 22, the day after the “apocalypse”. But NASA “was so sure” the world wouldn't end, they released the video more than a week earlier on December 11, 2012.
The video opens with: “"December 22, 2012. If you're watching this video, it means one thing. The world didn't end yesterday." NASA goes on to explain that the Mayan calendar is no different to the calendar that we use today.
"Just as the calendar you have on your kitchen wall does not cease to exist after December 31, the Mayan calendar does not cease to exist on December 21, 2012. This date is the end of the Mayan long-count period but then - just as your calendar begins again on January 1 - another long-count period begins for the Mayan calendar."
In a more recent video, NASA's senior scientist and astrobiologist, David Morrison reassures people by debunking several doomsday rumours.
“There's a recent internet rumour that NASA has confirmed three days of total darkness on the earth starting December 22. Well, NASA doesn't say that; nobody said it. It's just one more crazy idea.”
NASA has also posted on their website doomsday-related information explaining in detail why the world won't end.