What does it mean that the light shall not be clear?

Zechariah 14:6 And it shall come to pass in that day, that the light shall not be clear, nor dark:

  7 But it shall be one day which shall be known to the LORD, not day, nor night: but it shall come to pass, that at evening time it shall be light.

It also says that the light will not be dark, would that have been similar to the following?

Matthew 6:23b If therefore the light that is in thee be darkness, how great is that darkness!