EditorUtility.SetDirty not saving TimeSpan on ScriptableObject

I have a problem with my custom editor on a ScriptableObject.
When i edit the TimeSpan via my custom editor it doesn't save / i can't save it (Ctr + S doesn't do anything), meaning recompiling scripts, pressing start and closing + reopening of the project reverts its values back to the default.

Code (CSharp):

1. using System;
2. using UnityEngine;
3. [CreateAssetMenu()]
4. public class MySObj : ScriptableObject
5. {
6.     public TimeSpan timeSpan;
7. }

Code (CSharp):

1. using System;
2. using UnityEditor;
3. [CustomEditor(typeof(MySObj))]
4. public class MySObjEditor : Editor
5. {
6.     public override void OnInspectorGUI()
7.     {
8.         var mySObj = (MySObj)target; ;
9.         var timespan = TimeSpan.Zero;
10.         EditorGUI.BeginChangeCheck();
11.         timespan = timespan.AddDays(EditorGUILayout.IntField("Days", mySObj.timeSpan.Days));
12.         timespan = timespan.AddHours(EditorGUILayout.IntField("Hours", mySObj.timeSpan.Hours));
13.         timespan = timespan.AddMinutes(EditorGUILayout.IntField("Minutes", mySObj.timeSpan.Minutes));
14.         timespan = timespan.AddSeconds(EditorGUILayout.IntField("Seconds", mySObj.timeSpan.Seconds));
15.         if (EditorGUI.EndChangeCheck())
16.         {
17.             mySObj.timeSpan = timespan;
18.             EditorUtility.SetDirty(mySObj);
19.             AssetDatabase.SaveAssets();
20.         }
21.     }
22. }

I have tryed many things from other threads [Undo.RecordObject, EditorUtility.SetDirty, AssetDatabase.SaveAssets, AssetDatabase.Refresh etc.] but none of them worked in any combination i tryed.
I checked if EditorUtility.SetDirty sets the Obj to dirty by EditorUtility.IsDirty and it did.
Yes I know it would be easy to add a variable to MySObj like a double for hours and make the timespan a getter, but i would like it to leave MySObj as it is.

[not important for my problem]
The TimeSpan.Add functions are my extension functions:

Code (CSharp):

1. public static class Ext
2. {
3.     public static TimeSpan AddDays(this TimeSpan timeSpan, int days) => timeSpan.AddTicks(days * TimeSpan.TicksPerDay);
4.     public static TimeSpan AddHours(this TimeSpan timeSpan, int hours) => timeSpan.AddTicks(hours * TimeSpan.TicksPerHour);
5.     public static TimeSpan AddMinutes(this TimeSpan timeSpan, int minutes) => timeSpan.AddTicks(minutes * TimeSpan.TicksPerMinute);
6.     public static TimeSpan AddSeconds(this TimeSpan timeSpan, int seconds) => timeSpan.AddTicks(seconds * TimeSpan.TicksPerSecond);
7.     public static TimeSpan AddMilliseconds(this TimeSpan timeSpan, int millisecond) => timeSpan.AddTicks(millisecond * TimeSpan.TicksPerMillisecond);
8.     public static TimeSpan AddTicks(this TimeSpan timeSpan, long ticks) => timeSpan.Add(new TimeSpan(ticks));
9. }

 

Update: while looking throw the ScriptableObject File I've noticed that there was no difference between having a TimeSpan and not having one. Does this mean that a ScriptableObject can't "save" structs or non serialized classes?