John Calvin Commentary Exodus 6:13

"And Jehovah spake unto Moses and unto Aaron, and gave them a charge unto the children of Israel, and unto Pharaoh king of Egypt, to bring the children of Israel out of the land of Egypt." — Exodus 6:13 (ASV)

And the Lord spoke to Moses. I translate it, “the Lord had spoken to Moses;” because reference is here made to the commencement of his calling. Therefore, the sense will be more accurately rendered by the perfect past tense; for he repeats what he had already said: that he and Aaron his brother had not acted rashly, but had been commissioned by the command of God.

The main point is that, however often the work might have been in some way interrupted, the counsel of God still held firm for the liberation of the people. It is evident that he speaks of the first command, because he says that he and his brother were sent both to the children of Israel and to Pharaoh.