Moshe said, “Thus says Hashem: Approximately at midnight I will go forth among the Mitzriyim, and every first-born in the land of Mitzrayim shall die, from the first-born of Pharaoh who sits on his throne to the first-born of the slave girl who is behind the millstones; and all the first-born of the cattle. (Shemos 11/4-5)
Chazal (Brachos 3b-4a) understood that since Hashem knows everything, and Hashem is never uncertain about anything, Hashem certainly said to Moshe “B'Chaztos Ha’Laylah” "Exactly at midnight", and not “K’Chatozos Ha’Laylah” "Approximately at midnight, I Hashem will smite the first born.” This being so, when Moshe relayed the message to Paroah that Hashem will be bringing on the Plague of the First Born , why did Moshe change the words of Hashem and say, “K’Chatozos Ha’Laylah” "Approximately at midnight"? Chazal answer; Moshe was concerned that Paraoh’s advisors will make a mistake and think the plague did not happen exactly at midnight and they will say: Moshe is a liar.
Zera Shimshon asks numerous questions on this Gemara, we will focus on two of them. Firstly, after the Mitzriyim saw all of the prophecies that Moshe told to Paroah materialize and that Hashem answered Moshe's prayers to stop some of the plagues, why if the plague did not start exactly at midnight would Paraoh’s advisors call Moshe a liar? From all the previous plagues Paraoh’s advisors saw that Moshe was not a fraud. The advisors would realize that there is something wrong with their calculations and not that Moshe is a liar!
Secondly, why should the exact time of the plague detract from the effect of this miracle? Hashem brought all the plagues, including this plague of killing the first born, to show that Hashem is the sole king of the world and has full control of everything that happens. Why would people miss this point if it happened a little before or a little after Chatzos?
Background
1. Chazal tell us in numerous places in Shas that although all the millions of stars in the sky are stationary in relation to each other there are seven heavenly bodies that actually move. Each of these seven "kochavay lechess" (literally, "moving stars") "serve", meaning they have an influence on the world, in a different hour of the day. They follow one another in a continuous cycle in an exact sequence that began on the Wednesday of Creation and never changes. For instance, Tzeddek, Jupiter, always rules after Shabbtai, Saturn, and Ma'adim, Mars, always rules after Tzeddek etc.
According to this sequence, every Wednesday night, which was the time makkas b'choros took place, the hour before midnight "Tzeddek", Jupiter, rules and this planet fosters mercy, peace between nations and the stopping of feuds while the hour after midnight Ma'adim, Mars, rules and it has a leaning to war, loss and destruction.
2. At the exact moment of midnight "Tzeddek" no longer rules and "Ma'adim" has not begun to rule yet, so at that time the world is run only by Hashem without any assistance and influence from the stars and nature.
3. The Zohar points out that there were really two parts to the plague of the killing of the first born. One part was that Hashem showed His might by enacting His attribute of Justice and killing all the first born of the Mitzriyim. The second part was that Hashem showed His great mercy by saving the first born of Bnei Yisroel.
According to this, explains Zera Shimshon, Hashem told Moshe that He will smite the first born, and save Bnei Yisroel's first born, exactly at midnight for two reasons. Since no stars rule at that time Hashem will show that he is the sole ruler of the world and does not need the assistance of the stars. And secondly, that Hashem can punish and have mercy at the same time.
Moshe, however was concerned that Paroah’s advisors will make a mistake of the time and claim one of two things. They will either claim that the plague really took place before midnight when Tzeddek, "serves", which is merciful time. Because of this, even though by destroying the Mitzriyim, Hashem showed that He can execute justice even against nature but there still is no proof that Hashem can be merciful and save a nation without the help of the "stars".
The other possibility is that Paroah’s advisors will claim that the killing of the first born happened after midnight when "Ma'adim" ruled and we only have a proof that Hashem can be merciful and save Bnei Yisroel without the help of nature, however there is no proof that Hashem can punish independently.
In both of these scenarios Moshe would be called a liar, not because he simply made a mistake of a few seconds in predicting the time of the plague, but they will argue that Moshe lied with his claim that Hashem can do two contradictory acts at the same time without the help of nature and the stars.
Moshe therefore chose to be on the safe side and forgo the claim that Hashem can do two contradictory acts against nature at the same time in order for Paroah’s advisors to accept at least one claim. Either that Hashem can do something merciful against the laws of nature, if they claim it happened right after midnight, or that Hashem can execute justice without the assistance of the stars, if they claim it happened right before midnight.
