For what it's worth, if you don't feel like using the Ruby script but do have DFHack, you can simply go into the lua interpreter and enter this (and then let ten ticks pass) whenever you want to fix all existing creatures:
for key,my_unit in pairs(df.global.world.units.active) do if my_unit.relations.birth_time > 0 then my_unit.relations.birth_time = my_unit.relations.birth_time - (my_unit.relations.birth_time % 10) end end
(I hope I copied that down right; I reproduced it from memory just now, having fired off just that command myself a while back.) I can't really take credit for it -- I worked off of Putnam and Kurik's scripts, the explanations in the DFHack thread about what's actually going on and some online Lua references to boil it down to something I could merely type into the interpreter and get the effect needed. For what it's worth, you can confirm that it truncated the birth times of fortress-born creatures to the nearest-below ten ticks by running this before and after:
for key,my_unit in pairs(df.global.world.units.active) do print(key, my_unit.relations.birth_time) end
The Ruby script has the advantage that it should be able to be repeated regularly by DFHack and started up automatically on game launch. My method is simply a fairly easy way to go in and manually fire it off when you feel it would be helpful.
WARNING: I take no responsibility for screwing up your fortress if either of us makes a nasty typo. Double-check your work before sending it to the machine. Double-check my work before sending it to the machine.
And yes, I'm one of those annoying people who writes complex chains of shell commands on the fly to accomplish stuff that most people would look online for a program to do. (I suspect their way is faster, but my way means fewer programs to uninstall when I'm done. But I digress.)