hsa distributions to pay yourself back for medical expenses from prior years?

on 4/15/2019  i funded an HSA account for tax year 2018 . I have had several medical expenses since than but never used the debit card associated with that account (and thus never received any 1099-SA)

 

am I allowed to 'reimburse' myself NOW (in 2020) for all of those expenses I incurred during TY2018 or only for expenses incurred after the date I opened the account (4/15/2019) ?

 

when I complete my 2020 taxes, do these reimbursements have any impact on my taxes?

 

THx