When exactly in history did the Chachamim adopt the 24 hour day?
Our calendar goes by chalakim (which are sections of the amount of daylight), but when it comes to waiting a certain amount of hours it goes by hours based on the 24 hour cycle. Offhand, the only halacha we could think of that requires hours based on the 24 hour cycle is waiting from meat to milk. In that case you need to wait a certain amount of hours based on the 24 hours, rather than halachic shaos/chalakim.
Was it always like this? Wasn't the 24 hour day a relatively recent invention introduced with the Gregorian calendar? I've seen that it might go back to Egyptian times but I'm looking for good sources. Could it have been that waiting x number of hours between meals was at one point based on chalakim rather than hours from the 24 hour cycle?
Additionally, can anyone think of other halachic cases that use the hours from the 24 hour cycle to wait for something?