which will be purchase date we are considering.
As per my view, You consoder three date for purchase
Purchase order creation date,
Good receipt date
But as per my view actual purchase is when you receipt good at your plant.
Because if you create purchase order, then it is possibel your purchase is cancel or you have change the material or vendor.
So please consider good receipt date as purchase date.
Which you can see MIGO - Display - Doc info, here you can see created by and created on or posting date.