[keycloak-user] configure keycloak nbf value in jwt token

Hi, I am trying to use keycloak as our authentication server in the google cloud endpoints. But looks the google cloud endpoints required nbf > 0. In the jwt token I get from keycloak, the nbf is always 0. Is there a way I can configure the nbf value in the keycloak jwt token? I search the documentation and the internet but did not get any result. Thanks in advanced Xiaoling