How is player input handled and how to get the exact input time

Im not sure you can get the very exact time input was received, just because the nature of computers, there is always going to be a time gap between when input is received and the computer can respond. I think setting a variable as soon as input is received is about as close as you can get, so close in fact I do not think the time gap could even be humanly preceivable. And if that is not acurate enough, im courious to know why you would need greater accuracy.