The Hebrew word, "yowm," which is translated "day," actually means an unspecified period of time.
In
genesis, this word implies something different than the normal 24 hour period.
This
can be seen in verses Gen 1:14-19.
There,
on that fourth "yowm," solar time is itself created, and the 24 hour day is specifically enumerated among the other works
of the creation.
