I've tested with the latest uploaded version and it works just fine for me I'm thinking you may have a conflicting mod.

I will look into this as soon as I can, thank you!

Unfortunately also doesn't compile on b1.8.16 for me, also appears in Error category.

Thank you for the mod, it should be in the base game as an option, hint, hint

Could be that the ActorCustomization instance hasn't been initialized before your method is called. You can use Unity's coroutines to delay your method by a frame or 2.

The hacky method will break if I end up rearranging the UI in the future, but I'm pretty sure that won't happen.

I just did the hacky way lol.

I am working on it now and I remember why I couldn't do the slider everytime I would call ActorCustomization.Instance.DaysPerMonth.maxValue I would get this error in game System.NullReferenceException: Object reference not set to an instance of an object
at DaysOfMonth.DaysOfMonthModBehaviour.OnLevelFinishedLoading (UnityEngine.SceneManagement.Scene scene, UnityEngine.SceneManagement.LoadSceneMode mode) [0x00016] in <724c5ba410c5437586bec00920112476>:0
and after about 3 hours of trying anything to get it to work I just gave up. No fault to you or anyone else, I am just new to making plugins in general and going off the documentation I can find information for. Like with the starting years you don't have to call an instance it's just ActorCustomization.StartYears but for this it's within the instance of the game and I am dumb lol

Yooo thank you!! I will take a look into that in the next few days and update the mod accordingly, you're the goat!

A more hacky way to do it, but easier if you don't know where to look is to use the "UI_UNDER_MOUSE" command to find the path to the slider "MainPanel/GameConf/DaySlider", and then call WindowManager.FindElementPath("MainPanel/GameConf/DaySlider").GetComponent<Slider>().maxValue = 31

After loading the "Customization" scene, execute "ActorCustomization.Instance.DaysPerMonth.maxValue = 31".