1856 Bouviers Law Dictionary

NUNC PRO TUNC, practice. This phrase, which signifies now for then, is used to express that a thing is done at one time which ought to have been performed at another. Leave of court must be obtained 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. 3 Man. Gr. & Sc. 970. Vide 1 V.. & B. 312; 1 Moll. R. 462; 13 Price, R. 604; 1 Hogan, R. 110.
While your attention is occupied elsewhere something happens which you should have taken care of at the time. *This is not a fatal error that you must live with for Eternity. *While you cannot recover what you have lost as soon as you intone the spell NUNC PRO TUNC you immediately transform yourself back in time to the point where the trespass made itself clear and remedy is made.

This is the opposite of being considered served as soon as the notice letter is received by the post office. *You might receive the letter in a week or never but actual delivery is never the issue. *Instead it is the mailing date that is of importance.

Nunc pro tunc is a principle of Law. Equity appears to not have such a concept. Instead Equity requires that you pay penance for every little thing you failed to respond to within half a milliseconds of the event. And even if you did happen to fix the problem in half a millisecond Equity proponents would be quick to point out that the act was valid for that period of time and a precedent was established from which your descendants would suffer for eternity.