Hours of the day in Halacha

This is of course not explicit in the Torah, but this is what we have received in tradition. Is there a Jewish law in this or is this a norm that was practiced back then before there were clocks? It seems to me that the second is correct. And yet this is what is halakhically established and I see no reason to deviate from it. Regarding midnight, I really don't understand the logic. What has changed since then that moves us from sunset to midnight? This is a change in the law and not its adaptation to the reality of our lives. Is the Jewish calendar the one that is binding because it is customary to use it? At most, we can talk about the seven o'clock hour instead of the time that is defined by the priests who enter to eat their offerings.