42 CFR §424.15

1. (a)Medicare Part A pays for inpatient CAH services only if a physician certifies that the individual may reasonably be expected to be discharged or transferred to a hospital within 96 hours after admission to the CAH, and that the services are provided in accordance with § 412.3 of this chapter.
2. (b)Certification begins with the order for inpatient admission. All certification requirements must be completed, signed, and documented in the medical record no later than 1 day before the date on which the claim for payment for the inpatient CAH service is submitted.