It seems that in the process of removing the old milking system, I failed to notice that "person.lactating.daysunmilked" was only updated in the old system. There are currently no safe and easy methods to stop lactation. The only way that exists at the moment is the mutation spell.
There is not an exact match to that old system's code format so the update of that value is somewhat complicated.
The zero-ing of that value would need to be added to extractMilk in expansionFarm.gd.
person.lactating.daysunmilked = 0 #add var milkproduced = 0
The incrementing(+= 1) of the value would need to be added to dailyLactation in expansion.gd just after the code checking if they are lactating.
if person.lactation == false: return text person.lactating.daysunmilked += 1 #add
Note that the leading tabs are turned into spaces and I have excluded one layer of tabs, so you cannot simply copy this code into your file.
Also, if you use the leak spell on them, suck on their nipples or use a milker during sex it will reset the timer.