Things exist at the Big Bang therefore the act of creation cannot be at this point.
The act of creation is at (a). So that is an important event. You have two events one follows another. There is something which separate these two points from each other, time.
Definition time, since this is philosophical interchange, okay? If we are going to use a word together, let’s try to understand it together.
Many of us are using the word “Act” (i.e., the Act of creation).
For me, the meaning of Act is, “anything which is currently happening” (not something that potentially might be happening).
So, “the act of creation” for me equals “creation in its operation”.
I suspect some may mean by “the act of creation” that It is “The operation of the creator to make creaturely objects come into being”,
Two parallels might be:
The act of building equaling “the standing structure” (currently this matter of its makeup is existing with doors and windows and rooms and roof - the matter is ‘happening’ in this makeup)
or
The act of building can be seen as movement by carpenters happening, with the matter of their movement in potential to be the above first statement, “the act of building” (the happening finished structure, which, while it stands is a happening finished structure or Act).
The trouble with God is, when he is in his carpenter act, the house is complete simultaneously. - two Acts with no distinguishing of beginning, or one prior to the other in time.
While you need God as the carpenter, there is no pre-complete-house stage, like there is with earthly carpenters - and they step away from the house when an earthly house stands.
And yet, there is no complete house and no pre-complete-house without God thinking or speaking as the carpenter.
Even now he is my carpenter, and yours, in the act of creation and simultaneously we are acts of creation.