No. Only expenses incurred on or after your HSA establishment date qualify — which is why opening an account early matters even if you fund it with $1.
No. The expense must be incurred on or after the date your HSA was established. Anything before that date is permanently ineligible, regardless of how well you documented it.
This makes your establishment date one of the most valuable numbers in your financial life, and it creates a clear piece of advice: open an HSA as soon as you're eligible, even if you can only put a small amount in it. A funded account with a 2019 establishment date lets you reach back to 2019 expenses. An account you open next year does not.
A few states treat the establishment date slightly differently, and rules can vary if you're rolling over from an Archer MSA. Save a record of your account opening confirmation alongside your receipts — it's the document that anchors everything else.