Update November 8, 2017: The SegWit2X hard fork has been cancelled. The countdown clock below is no longer applicable but has been left up for posterity.
Bitcoin is upgrading (forking) on or around September 30, 2017, 3:46 am EDT based on the SegWit2X code on block height 494784. This is a simple count down timer that calculates the targeted fork block height minus the current block height multiplied by the Bitcoin 10 minute block time interval. This is a rough estimate and unfortunately not 100% accurate since not all Bitcoin blocks are found exactly in 10 minute intervals due to the time in between difficulty adjustments as explained here.

Timer Calculation: (hard fork block height - current block height) * 10 * 60