Priority Date
Here is the explanation of PD from
www.immihelp.com
Priority Date
The person can apply for adjustment of status when his/her priority date is current. This can be checked at online or calling the state department telephone at 202-663-1541 (recorded message). The web site and the telephone system are updated around 15th of each month. The priority dates are the Department\'s means of establishing a waiting list for U.S. permanent residency in those Categories where the number of applicants qualified for permanent residence exceed the annual limit specified by Congress.
Priority date is your place in line for getting the green card. If you have filed for alien labor certification, then your priority date is the date on which your application was received by the state level agency (SESA). If instead of labor certificate, you are going through the NIW/EB1 route, then your priority date is the date you filed I-140 petition. Your priority date is based on your country of birth and preference category(EB2, EB3 etc.) Currently priority dates for all the countries are current. But occasionally priority date for countries like India and China go back because they have huge backlog because of per country quota and huge number of applications from those countries.
Current priority date means there is no waiting involved. All persons qualified for the category in question can immediately apply for adjustment of status (if within U.S.) or an immigrant visa (if outside the U.S.).
Unless the person has filed adjustment of status, the person would have to leave US if his/her six year H1(5 year L1) limit is expiring. Please note that to count the six year limit, the total time spent on H1+H4+L1+L2 is counted. Please note that it is possible to get your H-1B visa approved for more than 6 years in some cases depending upon your employment based green card processing status. Please refer to the appropriate section on H-1B visa page to find out more how you can extend your H-1B visa for more than 6 years, either 1 year at a time or 3 years at a time.
If the priority date retrogates after filing the adjustment of status application, that application would be achieved at INS and would no longer be processed until the priority date becomes current again. So it is wrong to think that once priority date is current and adjustment of status is applied, priority date does not matter to you any more. Priority date always matters until you get your green card.