#unitytips: TimeSpan Box Drawer - http://diegogiacomelli.com.br/unitytips-timespan-box-drawer

TimeSpanBoxAttribute.cs

using UnityEngine;
using System;

namespace Giacomelli.Framework
{
    /// <summary>
    /// TimeSpan Box Attribute - http://diegogiacomelli.com.br/unitytips-timespan-box-drawer
    /// </summary>
    [AttributeUsage(AttributeTargets.Field)]
    public class TimeSpanBoxAttribute : PropertyAttribute
    {
        public TimeSpanBoxAttribute(string format)
        {
            Format = format;
        }

        public string Format { get; }
    }
}

TimeSpanBoxDrawer.cs

using UnityEngine;
using UnityEditor;
using System;

namespace Giacomelli.Framework
{
    /// <summary>
    /// TimeSpan Box Drawer - http://diegogiacomelli.com.br/unitytips-timespan-box-drawer
    /// </summary>
    [CustomPropertyDrawer(typeof(TimeSpanBoxAttribute))]
    public class TimeSpanBoxDrawer : PropertyDrawer
    {
        const float XPadding = 30f;
        const float YPadding = 5f;
        const float Height = 20f;
     
        public override void OnGUI(Rect position,
                                   SerializedProperty property,
                                   GUIContent label)
        {

            var attr = attribute as TimeSpanBoxAttribute;
            var value = TimeSpan.FromSeconds(property.floatValue);
            var text = FormatText(attr.Format, value);

            EditorGUI.PropertyField(position, property, label, true);

            position = new Rect(
                XPadding,
                position.y + EditorGUI.GetPropertyHeight(property, label, true) + YPadding,
                position.width - XPadding,
                Height);

            EditorGUI.HelpBox(position, text, MessageType.None);
        }

        private string FormatText(string format, TimeSpan value)
        {
           format = format
                .Replace(":", @"\:")
                .Replace(".", @"\")
                .Replace(@"0\:", "0:");

            return string.Format(format, value);
        }

        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            return EditorGUI.GetPropertyHeight(property, label, true) + Height;
        }
    }
}

TimeSpanBoxDrawerUsage.cs

using Giacomelli.Framework;
using UnityEngine;

public class TimeSpanBoxDrawerUsage : MonoBehaviour
{
    [TimeSpanBox("{0:mm} minutes and {0:ss} seconds")]
    public float TimeInSeconds;
}