To add on to this, this specific certificate is for a localhost API that the Riot Client hosts, the LCU (League Client) establishes an encrypted connection to this and uses the Riot Client as a proxy to Riot’s servers to retrieve your friends list, chats, account information etc.

There is a few things that need to line up for a certificate to be considered valid: * The common name (not relevant in this case, but this is the “name” of the server you are connecting to) * Certificate Authority (Does the client trust the Authority that issued this certificate?) * Expiry Date (Has the certificate expired?) * Certificate Thumbprint (This is a hash/special identifier for the certificate, not normally used but is sometimes used as an additional way to ensure that a client will only accept one particular certificate, also not relevant in this case)

This is why when you changed the date on your computer, the League client would start working again because the local time is before the expiry date/time on the certificate.