You can do it in NightZombies.xml

Set "Activate" to 1 (in global section)

them replace all:
<type name="Zmb
by
<type name="Z_mb
(use ctrl+H feature of your text editor)

as Z_mb... zombie class names don't exist the zombies shouldn't spawn at day time but when it is night time the "Night Zombies =" values (with Zmb... classes that still exist) will be used

lol ofc! That didn't even cross my mind for some reason! xD
Thank you!

So, I did that, and reset the server to make sure.
But they keep spawning during day, not as many but they're still appearing everywhere.
Not sure why?

Did you deactivate the vanilla zombies spawn system?

No, the vanilla is still active. But they don't have that spawn system have they? I get random groups of said zombies (mostly shirtless) out in the nowhere (not in towns/cities/etc).

Vanilla system shouldn't spawn zeds in middle of nowhere, but you should try to deactivate it, just to be sure.
Vanilla have no day/nigh system for zombie spawns.

Check if all zombie classes you use (those that still spawn the day) are configured in NightZombies.xml

I made the "regular" zeds slightly stronger, so they can take more than 1 headshot, and I notice the clear difference when they come from this mod, because you can kill them easily and they spawn randomly in the middle of nowhere. And it started happening after I added the mod (and for some reason it's still doing it even after I disabled the day-zeds).

But it's fine, I don't want to spend more time on this issue on my server :P Just thought I'd inform in case of possible need to fix something. Them being easier to kill, just makes it more fun having to avoid them (or kill them). Just thought it was weird, is all :)