A proper understanding of time helps us dispel many misconceptions about God's relationship to time, and our own.
For starters, God is not someone who's been sitting around "forever" on a timeline that stretches out infinitely before and behind us. If God was sitting on that infinitely-long timeline, He wouldn't be here at all, because neither infinity exists, nor do infinite sequences, and a timeline is a sequence.
You think you passed that one? What about perichoresis? There's a popular theory that the three persons of the Godhead are in a perpetual dance, and have been for an infinite period of time before choosing to create humans to join in the dance. The sentiment isn't far wrong, but the details are. God has not been dancing for an infinite period of time. "Infinity" does not exist. And neither do infinitely-long time periods.
So where did God come from? He didn't. He always existed. Something had to always exist. He just happened to be the one.
"So He's been around forever then? You're contradicting yourself!" Nay - He has not "been around forever" if you define "forever" as "an infinite time period leading up to the present moment". God never "came into existence" - He always was. But in His state of perpetual existence, He had a first moment.
God had a first experience.
But how could God have a first experience without a prior cause?
Revisit Time Type A : God was the cause of His own first experience. Not the cause of His own existence, but the cause of His own first experience.
So is God inside time, or outside? This is a common question, and one I pondered on for many years. Surely God is outside time I would think, for He cannot be inside time, locked to the same constraints which bind the rest of us.
In those days, I viewed time as a big box - like a shoebox, with God placed non-spatially outside the box.
But neither view is correct.
Time is not a box in which God is confined, nor a box in which God is not confined. Time (type B now) is the long sequence of events that has transpired from that first ever moment of time (time type A) - that first event in the universe, when God was the cause of the first moment in history.
View time as a long ribbon, ever extending in one direction, with God at the start of the ribbon and causing its extension.
So what about Heaven? So often I've heard preachers or Christians generally announce triumphantly that there'll be no time in Heaven. Baloney! No sun nor moon, ok, but there will be time. Scripture says there will be no sun nor moon, but it doesn't say there will be no time. In fact, it distinctly says that there will be time in Heaven - just not in those words.
Take, for instance, the marriage supper of the Lamb. Tell me, how do you plan to sup without time? How can you have a before state of food in bowl and an after state of a satisfied eater if there is no time? Time, as measured in earth years and days might long be gone, but time will exist, just measured differently (if measured at all - and I suspect it will be - consider for example that even in the perfect creation God gave stars for "signs and for seasons" - i.e. for tracking the passage of time).
Tell It Like It Is