[Question] Adding time limits for unit availability

[Hazbones]

I am making a mod and I want to know what is the best way to impose time limits on unit availability?

For example: a basic spear unit (early availability)
an improved spear unit (mid game available)
finally, an advanced spear unit (late game available only)

I don't want to attach the units to a building level because some provinces have high level bldgs from the start. I want the units to become available (then stop availability) based on the game time frame.

How would one go about making that happen?
I hope there is a simple column in a table somewhere one could assign the availability there...

Thanks in advance!

[Hazbones]

Anyone have an idea how to do this?

[Bad213Boy]

Can't you just put False in the Mid-Game Available column? I'm not sure what those columns actually do. I thought it might have something to do with beginning, realm divide, and then end. Ya those columns are weird. Wish there was more documenting on this stuff.