This problem is resolved. It turns out I had a hardware problem with my OSBo. I sent it in to Ray and he replaced a defective switching regulator. He found that the RTC worked just fine if the BBB was powered by 5VDC. However, when powered by 24VAC, it showed the symptoms I described. It turns out the switching regulator was outputting too much ripple voltage that affected the RTC. He replaced the switching regulator and the RTC is now functioning well under 24VAC.