The JWT token has a parameter exp to check the expiry time of the token. It would be nice to have also a parameter for the start time of the token validity.
After reading https://tools.ietf.org/html/rfc7519 I can express my wish more precisely:
Please implement the JWT claim “nbf” (Not Before) .
It is really funny, I can answer my question myself: the nbf claim works. It should be added to the example of the documentation: https://github.com/jitsi/lib-jitsi-meet/blob/master/doc/tokens.md