There is no regulation associated with Health Savings Accounts regarding when you pay your provider for medical care.  The IRS is not concerned as to when you pay your bill, their only concern is that you take withdrawals for medical expenses.  I would advise whenever you take a withdrawal from your Health Savings Account, you put a copy of the bill or Explanation of Benefits from the insurance company with your tax return. 

Also, withdrawals are based on the date of service to the medical provider.  So, as long as the Health Savings Account is opened prior to the date of service, then you can withdraw the funds at anytime.