The 1970 bug is a slightly annoying and mostly entertaining software glitch that bricks any iPhone by setting the date back before May 1970. Unsurprisingly, Apple's correcting that glitch in the next version of iOS. The most recent beta of iOS 9.3 fixes the bug by limiting the system time — you now can't set it back before 31 December 2000, stopping any possibility of bricking the handset. Apple hasn't officially said what caused the problem, but it's likely linked to the Unix time system used in places in iOS, which struggles to handle dates before 1970.
The bugfixed beta is already available to developers and public beta testers. The full software should be rolling out to general public sometime this autumn, and an educated guess would point to a launch sometime around Apple's March 15 event.