That is true, but that would be the definition of +. The OP is referring to the application of that operator to its arguments, which (depending on semantics) could also be a set of sets, or it could be a rewriting (ie, time based) process.
Time does not appear in formal logic or number theory, even though it is convenient to think of a derivation or a calculation as as sequence of steps performed one at a time.
it is also very convenient to use mathematics to model a temporal process, but we can, and often do, then show what happened (or will happen) as tables, graphs or other static representations.
I know that last sentence presupposes time, but I am not even going to attempt to rewrite it in a way that avoids doing so!