HSA Distribution - Date is Service in prior year. Received bill in current.

 Hello, in December of last year I incurred about $500 in medical charges. However I did not receive my bills until January 2018. I opened up an HSA account on January 1. Am I eligible to use this account to pay for expenses that I incurred last December since they were billed after I open the account. Or since date of service is last year they are not qualified?