from wikipedia.org
General :
Definition
Nunc pro tunc or now for then is a phrase which theoretically applies to acts that are allowed to be done after the time when they should have been done with a retroactive effect. However, the practice is often used to cover up misdeeds or to make the past disappear in Orwellian fashion so that a new record of the past can be inserted in place of the original record. In other words it is supposed to be an inserted entry made now of something actually previously done to have effect of a former date. The idea is that the corrected record will not create a new record, but create a true record that should have been made originally, but due to inadvertence or mistake, was not done at the time.
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Immigration:it may mean a benefit given with affect from retroactively.
Example of a case whose wife is already out of status,which explains nunc pro trunc.
:from Chang&Boos:
http://americanlaw.com/q&a99.html:
According to INA §248, a prerequisite for a change of status is that the alien continue to maintain status. As applicant's wife has already overstayed, she is not eligible for a change of status. The only way that applicant's wife will get a change of status is if he request nunc pro tunc (retroactive) approval of an extension of stay. The INS regulations provide for nunc pro tunc grants of extensions of stay only in limited circumstances. According to 8 CFR §214.1(c)(4), failure to file before the period of previously authorized status expired may be excused in the discretion of the INS, without the need to file any additional application, where it is demonstrated at the time of filing that:
The delay was due to extraordinary circumstances beyond the control of the applicant or petitioner, and the Service finds the delay commensurate with the circumstances;
The alien has not otherwise violated his or her nonimmigrant status;
The alien remains a bona fide nonimmigrant; and
The alien is not the subject of deportation proceedings under section 242 of the Act (prior to April 1, 1997) or removal proceedings under section 240 of the Act.
Whether applicant will be able to show that his wife's failure to make a timely filing for an extension of stay was due to extraordinary circumstances beyond her control is unknown. However, the reasons he has given for the delay do not sound like extraordinary circumstances beyond his wife's control. There is also the additional requirement that the delay must have been commensurate with the circumstances; 180 days is a long time so you will have a tough time arguing that the delay was reasonable. Nevertheless, it probably cannot hurt to apply for nunc pro tunc approval of his wife's extension of stay, as long as she does not remain unlawfully present for 12 months. If she does, a ten year bar will apply to her instead of a three year bar.
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Not a leagl opinion.