Psalms 90:4

For a thousand years in thy sight are but as yesterday when it is past, and as a watch in the night.

A thousand years are brief in God's sight, like a day that has already passed. They are also like a watch in the night, a short portion of darkness soon gone. This brevity stands set against God's everlasting existence, which has no such limit or passing. By that contrast the thousand years show how short human life is.