halve <iMonthIncrement> (how many months each turn is)
double <iTurnsPerIncrement> (how many turns game uses this month amount increment type)
iMonthIncrement must be full years ie. number that can be divided with 12 when turn is longer than 1 year. Game crashes if you for example make 10.5 year turns, but you can have game turns with less than 12 months without problems.
Example:
You got 2000 year period and you have 200 turns each 10 years and want to double turn amount for this:
<iMonthIncrement>120</iMonthIncrement> (12 month * 10 years = 120 months)
<iTurnsPerIncrement>200</iTurnsPerIncrement> (how many turns)
result would be
<iMonthIncrement>60</iMonthIncrement> (12 months * 5 years = 60 months)
<iTurnsPerIncrement>400</iTurnsPerIncrement> (how many turns)
5years * 400 turns = 2000 years
One problem you might run into is the case when game turn length is 25 years and you want to halve this ie. you'd get 12.5year turns. 25*12 = 300, half is 150 which can't be divided with 12. So you'll have to make slight adjustments and instead of making 12.5 year turns, you just make 12 year turn but add some more turns to compensate the month difference or otherwise your modded timeline ends the game at wrong year.
Example 2000 year period with 25year turns:
<iMonthIncrement>300</iMonthIncrement> (12 month * 25 years = 300 months)
<iTurnsPerIncrement>80</iTurnsPerIncrement> (how many turns)
result would be:
<iMonthIncrement>120</iMonthIncrement> (12 month * 12 years = 144 months)
<iTurnsPerIncrement>167</iTurnsPerIncrement> (how many turns)
(2000*12/144=166.66666... ~167 turns, 167*12years = 2004 years)
This would make the period last 2004 years which you would have to correct somewhere else, for example on period where turns last for 1 full year by reducing turn amount by 4 there.