Looks like we ended up with this calendar.
Since we don't display days and months beyond 1800 - 2200 AD time frame, I guess I can use day increments anywhere except Prehistoric (already used in 80% of all cases)
Era time frames and duration:
Prehistoric: 200 000 BC - 12 000 BC (188 000 years) // 50 000 BC would be at ~5% of game.
Ancient: 12 000 BC - 1200 BC (10 800 years)
Classical: 1200 BC - 450 AD (1650 years)
Medieval: 450 AD - 1400 AD (950 years)
Renaissance: 1400 AD - 1750 AD (350 years)
Industrial: 1750 AD - 1900 AD (150 years)
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Modern: 1900 AD - 1990 AD (90 years)
Information: 1990 AD - 2050 AD (60 years) --- Shortest time step here.
Nanotech: 2050 AD - 2150 AD (100 years)
Transhuman: 2150 AD - 2300 AD (150 years)
Galactic: 2300 AD - 3000 AD (700 years)
Cosmic: 3000 AD - 4000 AD (1000 years)
Transcendent: 4000 AD - 6000 AD (2000 years)
OLD calendar:
Prehistoric: 200 000 BC - 6000 BC (194 000 years), 190 000 / 4000 years in first/last part.
Ancient: 6000 BC - 2000 BC (4000 years)
Classical: 2000 BC - 600 AD (2600 years)
Medieval: 600 AD - 1300 AD (700 years)
Renaissance: 1300 AD - 1700 AD (400 years)
Industrial: 1700 AD - 1900 AD (200 years)
----
Modern: 1900 AD - 1990 AD (90 years)
Information: 1990 AD - 2050 AD (60 years) --- Shortest time step here.
Nanotech: 2050 AD - 2150 AD (100 years)
Transhuman: 2150 AD - 2300 AD (150 years)
Galactic: 2300 AD - 3000 AD (700 years)
Cosmic: 3000 AD - 4000 AD (1000 years)
Transcendent: 4000 AD - 6000 AD (2000 years)