The date is a guesstimate. In times of persecution Bunyan is reputed to have preached under the Oak. He was arrested only a mile away in 1660 so it is likely that this story is true.
From Harlington take the Barton Road and you will pass St Mary's Church on your right and then there will soon be open countryside on your right. Continue on until you come to a road to the left called Goswell End Road; park there. Having parked walk back to the Barton Road and turn left. Walk approximately 70 yards and on your left you will see a farm gate. Just past the gate there is an entrance to a footpath next to a sign-post. Walk along until you come to the field and with your back to the path you will see Bunyan's Oak down and to your right (at about 55 degrees).