Get end-date from .pem encoded file with openssl

openssl x509 -enddate -noout -in ACERTFILE.pem
>>> notAfter=Aug 28 23:59:59 2021 GMT

Also, you can get if a certain certificate is already expired or not (with 2 minutes (120 seconds) span) 

openssl x509 -checkend 120 -noout -in ACERTFILE.pem