Sibly is right, her is the language from Prop. Treas. Reg §1.125-6(a)(2)
Medical expenses incurred before the later of the effective date of the plan and the date the employee is enrolled in the plan are not incurred during the period for which the employee is covered by the plan…[Medical] expenses are incurred when the employee (or the employee's spouse or dependents) is provided with the medical care that gives rise to the medical expenses, and not when the employee is formally billed, charged for, or pays for the medical care.