here's an idea
have a command that at every lets say 50/100 turns verifies that the number of turns to years ratio until the next era advance is good and if not adjust it
example: lets say classical era has another 500 years or so, the number of turns necessary to reach the first tech that advances the era is 100, the command calculates the year/turn ration 500/100 and sets 5 years/turn
at the next calculation after 50 turns their will be 250 years left, but lets say you have an big increase of research and now you are 25 turns till nearest tech who era advances, the command calculates again 250/25 and sets 10 years/turn
at every era advance the command calculates again the ratio so you wont have lets say 1000 years/turn going from prehistoric to ancient
have a command that at every lets say 50/100 turns verifies that the number of turns to years ratio until the next era advance is good and if not adjust it
example: lets say classical era has another 500 years or so, the number of turns necessary to reach the first tech that advances the era is 100, the command calculates the year/turn ration 500/100 and sets 5 years/turn
at the next calculation after 50 turns their will be 250 years left, but lets say you have an big increase of research and now you are 25 turns till nearest tech who era advances, the command calculates again 250/25 and sets 10 years/turn
at every era advance the command calculates again the ratio so you wont have lets say 1000 years/turn going from prehistoric to ancient