Where did the duration of a week come from?

We are attending a series of lectures by the Institute for Creation Research (ICR), offered at our church in the evenings... the series actually started with Dr Jason Lisle teaching during the service this morning.... he brought up an interesting concept:

In general, time periods are determined by natural phenomena - a day is the amount of time for the earth to rotate once, a year is the amount of time for the earth to orbit the sun, a month is the amount of time for the moon to orbit the earth.

Where did the duration of a week come from?

From the account of creation in the Bible!

Genesis 2:2-3

By the seventh day God had finished the work he had been doing; so on the seventh day he rested from all his work. ³ Then God blessed the seventh day and made it holy, because on it he rested from all the work of creating that he had done.

or Exodus 20:11

For in six days the Lord made the heavens and the earth, the sea, and all that is in them, but he rested on the seventh day. Therefore the Lord blessed the Sabbath day and made it holy.