The final rule allows such F-1 STEM students who have elected to pursue 12 months of OPT in the United States to extend the OPT period by 24 months (STEM OPT extension). This 24-month extension effectively replaces the 17-month STEM OPT extension previously available to certain STEM students.

Dates: This rule is effective May 10, 2016, except the addition of 8 CFR 214.16, which is effective from May 10, 2016, through May 10, 2019.

https://federalregister.gov/a/2016-04828