What are the things to understand when we say that God is eternal?

Time is a system of measurement marked by a succession of events in a changing universe. Because the immutable God exists outside his transient universe, it is necessary to say that he also exists outside of time as we know it. Time began when God created it.

Though some of the passages above convey something of a temporally linear sense, reflecting the vantage of the human author, these should be regarded as an instance of anthropomorphism. God is not merely everlasting (i.e., he exists at all times); he is also eternal (i.e., he exists outside and above time).

That God does not regard time as we do (see, e.g., Ps 90:4; 2 Pet 3:8) does not mean that he cannot interact with temporal beings or that he is careless in his expression of temporal realities. That a day is like a thousand years to God and a thousand years like a day does not mean that God is incapable of differentiating between a day and a millennium or is capricious in his use of temporal designations.