An analog for understanding omnitemporality is omnipresence. Both attributes together speak to the Creator's relationship to space/time, and as omnipresence has been fairly well understood and defined, so too can omnitemporality be understood and defined. In relation to space, God's omnipresence is such that every point in space is before God--he is everywhere present at once. In relation to time, God's omnitemporality is such that every point in time is before God at once.
Time began with the beginning of creation, and only has meaning within that creation. Outside of creation, there is no before since time does not exist there, there's just is. To apply a time construct to God; for instance, "God did this and then he did this before he created," is to attempt to apply a descriptor that has no meaning in the situation to which it is being applied. Looking out from creation (and time), one could say, "before creation," meaning outside of creation, not within the framework of time, but that one would err if they meant to extend the timeline before creation.
Such attempts to impose time outside of creation can only lead to confusion and inaccuracy in describing or understanding God or his thoughts and actions apart from creation. Time is contained in the bubble of thought in the mind of God which is creation. It has no impact nor meaning outside that bubble. It will only have existence within that bubble as long as that bubble is maintained.
When that bubble is dissolved and replaced a new regimen is established, it may or may not have a quality like time. Regardless, as for God, He is.