Under FMLA, you can designate retroactively if you provide appropriate notice to the employee and there is no harm to the employee. You and the employee can also mutually agree to designate FMLA qualifying leave retroactively.
Under CFRA, employers can't retroactively designate leave as CFRA leave after the employee has returned to work except with appropriate notice to the employee and where the employer's failure to timely designate does not cause harm or injury to the employee.
Read more about Family and Medical Leave Designation in the HR Library.