Your "date acquired" is the same date as the date the stock became unarguably and unequivocally "yours", to do with as you wish.
It's not the "grant date", typically, because usually the grant date doesn't mean that you really "own" anything.
Typically something has to happen - a RSU has to "vest", a NQSO has to be "exercised", etc. - before you really and truly own the stock. So for common "option" - a right to by so much stock at a fixed price - it's the exercise date that's the acquisition date. For restricted stock, RSAs and RSUs it's the vesting date that's your acquisition date. For ESPPs it's the date that the pot of money that's been accumulating from your for some number of months is actually used to buy the stock.
Tom Young
