Starting a new epoch every 50 years is kind of round and leaves plenty of space in between. 1970 is epoch 1, so this one will be epoch 2 and the next one in 2070 will be epoch 3 etc. It will be easy in a few hundred years to know which epoch is the current one. 2020 not 2000 for backwards compatibility to keep the intervals even for the new epochs, because the first one is already baked everywhere so we can't switch it to 1950.