What is a Serial Number? A serial number for an X509 certificate is a number assigned by the issuer that distinguishes the certificate from other certificates. A valid serial number contains a value between 1 and 2147483647. What is a Validity Period? A validity period for an X509 certificate is the number of days the certificate is valid from the time it is issued. Certificates issued by Certificate Assistant have a maximum validity period of 20 years. EFTA00289901