According to Wikipedia, concepts superficially similar to the modern system of hours have been around since at least ancient Egypt c. 2800 BC. But their system divided "daytime" (after dawn and before sunset) and "nighttime" (after sunset and before dawn) into 12 hours each, whose length would of course vary at different times of the year.
At some point in the intervening ~5000 years, we moved to a system of a day being composed of 24 equal hours that daytime or nighttime could take up a varying quantity of at different times during the year. But I'm having trouble determining at what point this concept first took hold.
There's a related question on here that was closed for being confusing and unclear what exactly was being asked, so to avoid the same fate, here's exactly what I'm asking: when (and by who) was the system established in which each day has 24 hours, each of which are supposed to be of identical length rather than being dependent on varying values such as the length of time that the sun is in the sky?