A strip of the landscape of Minecraft was dark .  Anything that entered that strip also became dark. Light in that area was listed as zero and stayed the same during day and night.  Problem was found while in creative mode, but stayed the same when I switched to survival mode.  Exiting and re-entering game caused the problem to go away.  The location of the problem was at x=1489 to 1502, z=705 to 734, y= ground level to 94.  Above 94 the light level went back to normal.  While flying across the landscape I saw similar strips of darkness in other locations.