From my perspective, eternity cannot work both ways. God created time, existing outside of its limitations, therefore time has a definite beginning. You could say God was eternal before time existed, but what do words like “eternal” and “before” mean if time does not exist? Our perception is far too limited to conceive these ideas and speak about them in a rational or sensible way.
In mathematics, in straight line situations, the concept of infinity works both ways; no beginning and no end.
When you think about it, time is a man created concept that makes sense; it's a past and future straight line situation with no plausable beginning or end. So far as math and science is concerned, time is eternal.