“The amendment made by subsection (a) [amending this section] shall take effect on the date of the enactment of this Act [Jan. 28, 2008], except that, in the case of a leave recipient who is undergoing medical treatment on such date of enactment, section 6333(span)(2)(B) of title 5, United States Code (as amended by this section) shall be applied as if it had been amended by inserting ‘or the date of the enactment of this subsection [probably should be “paragraph”], whichever is later’ after ‘the start of such treatment’.”