29 months actually - from the typical graduation filing date of April 1 to the H1B validity start date of October 1 two years later. This 17 months extension only applied to STEM majors last time I checked. It was introduced in the spring of 2009 per DHS executive order (memo).

EDIT: Apparently the duration was changed recently. See comment below.

It used to be 17 months. Now it is 24 months. This is a new rule which started this year in March.

>On March 11, 2016, the Department of Homeland Security published a final rule allowing certain F-1 students who receive science, technology, engineering, and mathematics (STEM) degrees, and who meet other specified requirements, to apply for a 24-month extension of their post-completion OPT. The 24-month extension will replace the 17-month STEM OPT extension previously available to STEM students (see 73 FR 18944). Eligible students may now apply for a 24-month STEM OPT extension.

https://www.uscis.gov/working-united-states/students-and-exc...

Thanks! I stand corrected!

Does that mean the H1B validity now also starts before October 1?

The 17 month extension would have ended October 1 (traditionally the H1B start date). So I am curious what happens if one applies for and is granted the H1B in their third year under OPT.

I don't think so. But one can still use H1B-Gap which basically allows you to stay beyond 24 months if you got an H1B Visa. For example, if the 24 months are over in May, you are allowed to stay until October.