Though the intervals change in the first turns of Civ II, the last 170 turns always begin in 1850, and are yearly until 2020. They year 2019 is the last playable, scored year.
When any civ launches a SS, the turns automatically switch to yearly (no matter what the interval "should" be.
Between 1750 and 1850, all levels have an interval of 2 years.
In the "Time Warp" discussed in Duke's link, remember that the figures quoted are for a "normal" sized map. Choosing a small or large map can also affect the total number of game turns (large map = more game turns), and thus affect the intervals before 1750. BTW, All deity games are 420 turns, and all Chieftan games are 570 turns.