start gazebo simulation with non zero clock time

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When gazebo start the simulation it publish clock time to /clock topic. Some package use that time as their timestamps. Is there a way to tell gazebo to start counting from another time? px4 firmware in SITL stamps their own time in their messages and for some reason (I think that is becouse of their simulated GPS) it start stamping their message from time 633437442. I need to time synchroize this package with another that use gazebo clock as time reference.

Thank!