Not exactly an era, it's just the time when Terra Nova wasn't created. Dated between sometime in late 2016 until October 19, 2018 when Terra Nova (AKA EMC2.0, TN, or EMC Towny) was opened to donators. Server still used up until November 1, 2018 when Terra Nova opened for all and used intermittently until it went offline. Now you can visit it in spectator mode by doing /joinqueue classic. EMC Classic history is broken into 4 Epochs and 11 Eras.
Link to EarthMC Classic Wikia: Home Page