Really this is an impossible question.

As humans, we can't possibly know for sure, but judging on evidence, I would say that when the universe first started. But seriously, there are things we can't know.

The true beginning of time is when cake was invented

Well time is a construct of the human mind to explain the progression of change. Time didn't start until humans assigned it a name. What we think of as time is a series of events happening in sequence. ohh try to find the Quantum theory of immortality.