It's not a parable.
Is a thousand year a day? Or do we compare the temporal seen (day) to the eternal not seen (thousand)? The law of faith.
One day equals a unknown as the spiritual understanding of parable/prophecy.. understanding hid from temporal man seen .
Matthew 13:34 All these things spake Jesus unto the multitude in parables; (prophecy) and without a parable (prophecy) spake he not unto them:
Mark 4:11 And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables:(prophecy)
We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts: Knowing this first, that no prophecy of the scripture is of any private interpretation. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.1 peter 1: 19-20