13.7 billion years ago, the universe suddenly had a low enough energy density that we could understand it, and we call this the big bang.
However, we are in the thrall of several gravity fields. The Earth, The Sun, The Milky Way, The Local Group... even things at the same hierarchical level like Jupiter and the Moon affect the state of the gravity field where we are.
According to general relativity, gravity affects time. And with my, admittedly limited, understanding of it we move through time "slower" from the point of view of a satelite, and they move "faster" in time from our point of view.
I am assuming that the at least one way we date the age of the universe is something along the lines of measuring the distance to the CMB. So if we measure the light from the CMB as being 13.7b light years away, then the universe is roughly 13.7b years old.
My question is this. Since dark energy and gravity have been changing their relative strengths across the whole universe for this time, and since we are currently in a gravity field, and since the units we use for time were concocted IN said gravity field; from who's perspective, and with what definition of "year", is the Big Bang 13.7b years in the past and how does the changing gravity of the universe distort that value?