Nunc pro tunc is a Latin term meaning "now for then". It refers to a thing that is done at one time which ought to have been performed at another. Permission must be sought from the court to do things nunc pro tunc, and this is granted to answer the purposes of justice, but never to do injustice. A judgment nunc pro tunc can be entered only when the delay has arisen from the act of the court.
It also refers to changing back to an earlier date of an order, judgment or filing of a document. Such a retroactive re-dating requires a court order which can be obtained by a showing that the earlier date would have been legal, and there was error, accidental omission or neglect which has caused a problem or inconvenience which can be cured. Often the judge will grant the nunc pro tunc order ex parte (with only the applicant appearing and without notice). Examples: a court clerk fails to file an answer when he/she received it, and a nunc pro tunc date of filing is needed.