We see nations, people's lives, and plans mapped out on straight lines from left to right. We see a beginning and an end. He has no beginning or end. He simply has always been. Also, physics has shown that time is a property that is the result of the existence of matter.
Time exists when matter exists.
Time has even been called the fourth dimension. But God is not matter. In fact, God created matter. He created the universe. So, time began when God created the universe.