; and (B) shall provide the employer with not less than 30 days' notice, before the date the leave is to begin, of the employee's intention to take leave under such subparagraph, except that if the date of the treatment requires leave to begin in less than 30 days, the employee shall provide such notice as is practicable. . (f) Certification Section 103 of such Act ( 29 U.S.C. 2613 ) is amended by adding at the end the following: (g) Certification relating to bone marrow or blood stem cell donation leave An employer may require that a request for leave under section 102(a)(5) be supported by a certification issued by a contractor of the registry functions of the C.W. Bill Young Cell Transplantation Program. .