please god let me just use datetimes instead of unix timestamps

UDateTime.cs

using System;
#if UNITY_EDITOR
using UnityEditor;
#endif
using UnityEngine;

// we have to use UDateTime instead of DateTime on our classes
// we still typically need to either cast this to a DateTime or read the DateTime field directly
[System.Serializable]
public class UDateTime : ISerializationCallbackReceiver {
    [HideInInspector] public DateTime dateTime;

    // if you don't want to use the PropertyDrawer then remove HideInInspector here
    [HideInInspector] [SerializeField] private string _dateTime;

    public static implicit operator DateTime(UDateTime udt) {
        return (udt.dateTime);
    }

    public static implicit operator UDateTime(DateTime dt) {
        return new UDateTime() {dateTime = dt};
    }

    public void OnAfterDeserialize() {
        DateTime.TryParse(_dateTime, out dateTime);
    }

    public void OnBeforeSerialize() {
        _dateTime = dateTime.ToString();
    }
}

// if we implement this PropertyDrawer then we keep the label next to the text field
#if UNITY_EDITOR
[CustomPropertyDrawer(typeof(UDateTime))]
public class UDateTimeDrawer : PropertyDrawer {
    // Draw the property inside the given rect
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label) {
        // Using BeginProperty / EndProperty on the parent property means that
        // prefab override logic works on the entire property.
        EditorGUI.BeginProperty(position, label, property);

        // Draw label
        position = EditorGUI.PrefixLabel(position, GUIUtility.GetControlID(FocusType.Passive), label);

        // Don't make child fields be indented
        var indent = EditorGUI.indentLevel;
        EditorGUI.indentLevel = 0;

        // Calculate rects
        Rect amountRect = new Rect(position.x, position.y, position.width, position.height);

        // Draw fields - passs GUIContent.none to each so they are drawn without labels
        EditorGUI.PropertyField(amountRect, property.FindPropertyRelative("_dateTime"), GUIContent.none);

        // Set indent back to what it was
        EditorGUI.indentLevel = indent;

        EditorGUI.EndProperty();
    }
}
#endif