i can read the time through serial monitor on the Uno, but not on the Mega?
You can't read something on the serial monitor that the Arduino didn't write. It is silly to talk about what YOU can, or can not do. We are here to help you with what the Arduino can, or can not, do. SO, describe THAT.
You are aware, aren't you, that the SDA and SCL pins are in different places on the Mega and the UNO?