What evidence is there that time zero ever existed?

Great question! There is no evidence that time started with the Big Bang. Actually, there are several respectable hypotheses that describe the Big Bang as happening in space and time that was already there. Check out the Chaotic Inflation theory: http://en.wikipedia.org/wiki/Chaotic_inflation_the...

If the universe is expanding (btw; Hubble proved it is) then it is reasonable to assume it is expanding from a single common point, i.e, the Big Bang.

Since time is a measurement of entropy (orderly to chaotic, a basic principle of quantum physics) in the universe then it is safe to assume there was no "time" before there was matter to begin entropic degradation.

Hence the term "space/time" since the two concepts are inseparable. One cannot exist without the other. You cannot have a physical existence without a temporal existence and vice-versa.

The universe (all reactions simultaneously) may still be equal zero.

At the moment of the Big Bang, the mass of the whole universe was concentrated in a single point (singularity). The property of singularities is that the gravity gradient is such that the flow of time stops.

Since time advances now, then it must have started flowing at some point.

the big bang and those birth of universe and death are hypothetical which is not proved but seems to be true

There isn't, and it is theorized there isn't.

As you progress further and further toward the Big Bang, time becomes a "spatial" dimension in quantum foam.

There isn't. The physics we have today do not hold up that close to the singularity.

There is evidence that time zero is when you farted..... believe me ? can be proved ? everyone is dead ? so who is there to verify the eveidence ?

There is no direct evidence. You could call it circumstantial evidence.