When Does a Project really start?

Interesting, b/c officially, the project starts when approved. But I guess, its more a matter of perspective. Additionally, the kickoff is post all planning activities. Again, perspective. Officially, as per PMBOK, the kickoff is post planning, but there is also the kickoff with the project team.

It's somewhat difficult to say when does the project start, unless you have a clear idea of the project, an agreement that the project is worth undertaking, resources to work, and the approved budget.

All milestones of Project are mentioned in contract, PO or some kind of document it include start date, completion dates, intermediate mile stones. In case project Is internal then project charter address project start date else Kick off meeting date is considered as project start date but project start date must be agreed and documented, else project start date will be mystery on one will know and all will have their own interpretation.

As per PMBOK, the project is officially initiated when the project charter including business case, initial funds and time requirement has been approved.

I think it's when the project is "approved" whenever that is, and whatever that looks like. For some organisations/methodologies it could be at Project Charter stage, others at business case, others when the CEO says that it's going to happen :)

In Construction Projects, the project really starts when the charter, business case has been approved and funds allocated. Kick-Off meeting usually occur during the first month month when we officially hit the ground.

In my workplace, Elizabeth's answer is most appropriate. when the GM says "Yes"

IMHO, we need to qualify the term 'starts'. If we consider 'start' as a counter-point to 'ends', then would reckon, the project 'starts' when the project management plan is approved by the project governance and resources get allocated - atleast for the first stage, so that the project manager can commence the first stage work.
Of course, we know lots of work gets done before the project management plan gets approved, but this is more of a planning work - rather than getting into implementation.