A control circuit, a time calculating unit, and operating method for control circuit are disclosed. The control circuit is operated in a power converter and coupled to a load. The control circuit includes an output stage and a time calculating unit. The time calculating unit receives a control signal and a reference voltage and provides a switch conducting signal to the output stage. The generation of the control signal is related to an output voltage of the power converter. When the difference between the control signal and the reference voltage becomes larger due to the change of the load, the time calculating unit dynamically increases an on-time of the switch conducting signal.