Timing Fifteen Minutes with Two Hourglasses

You have a 7-minute hourglass and an 11-minute hourglass, and you must time a process that takes exactly 15 minutes. You may start the process only at the moment you begin timing, and you may flip either hourglass whenever it runs out (or at any time). Using only these two hourglasses, what is the least total elapsed time in which you can mark off exactly a 15-minute interval starting from time zero?