Let us extend this thought a little further. Pharaoh also knew about the decree of the 400 years of bondage and that they were still missing another seventy years. Therefore, he refused to let them out earlier. In fact, he remembered his dream that Yosef interpreted for him that the seven fat cows were devoured by the skinny ones. When he revisited that dream, he realized that he was presently experiencing the symbolic dream in real time with Moshe in the name of Hashem demanding to set the Jewish Nation free when their time is not yet over.
In Pharaoh's dream, he saw healthy full פרות cows which are spelled רפ (gematriah 280) and a ת which equals 400. This paralleled exactly the heavenly decree to fulfill 280 out of the 400. However, Yisrael are likened to the seven skinny lean cows who are lacking in the fulfillment of the seventy years, and it’s a chutzpah that they are demanding me to let them go prematurely as symbolized by the lean cows eating the fat ones.
Even Pharaoh's name suggested that the conditions of the bris has not yet been fulfilled. In his name is פר and also an ע which is the seventy years missing. These numbers made him obstinate in sending Yisrael free, a middah which is also found in these letters of his name - ערף which refers to stiff necked and unbending.
