This is because coring time is based on the time left till the game ends as I understand it, i.e coring goes faster the longer into the game you are. Safe to say there is a very long time until you get into the regular 400 year span of the normal game.

Go to the mods common folder and find the defines.lua and modify these files as you see fit. CORE_TIME_SIZE_MODIFIER = 0.04, (Core time increase per province owned)
MONTHS_TO_CORE_MAXIMUM = 240, (Maximun core time)
MONTHS_TO_CORE = 36, (Standard core time. Basis for all calculations)

I usually drag down the values somewhat. The easy fix to this is simply to set the max month to 10-16 months. This is of course not an optimal solution but it's what we have at the moment. I don't know anything about programming but I guess changing the possibly hardcoded core formula is a tad difficult.

thank you i wil wil see further into it

Seems to be fixed, I'm getting 18 months on Novgorod's provinces in the 1400's.

is this up to date?

Is there still a coring problem with the up to date mod?