According to the Maya calendar, time is running out: When the 5,125-year cycle ends on Dec. 21, so does the world. Or, if you don’t believe that, would you consider worrying about an interplanetary collision on that date? A solar storm? And what does it say about us that we’re willing to entertain so many Dec. 21 doomsday theories? Fortunately, scientific evidence shows that there’s nothing to worry about. Yet.