Day Night Cycle

TDD Contract

[Test]
public void TimePasses_AccordingToTimeScale()
{
    var timeSystem = new TimeSystem(dayLengthInSeconds: 60f);
    timeSystem.Tick(15f); // Advance 1/4 of a day
    Assert.AreEqual(6f, timeSystem.CurrentHour); // 6 AM
}

[Test]
public void TimePassesMidnight_RaisesDayPassedEvent()
{
    var timeSystem = new TimeSystem(dayLengthInSeconds: 60f);
    timeSystem.SetTime(23f); // 11 PM
    
    bool eventRaised = false;
    timeSystem.OnDayPassed += () => eventRaised = true;
    
    timeSystem.Tick(5f); // Pass midnight
    
    Assert.IsTrue(eventRaised);
    Assert.AreEqual(1f, timeSystem.CurrentHour); // 1 AM
}

Core Implementation

// --- TimeSystem.cs ---
using System;
using UnityEngine;

public class TimeSystem : MonoBehaviour
{
    [Header("Settings")]
    [Tooltip("How many real-world minutes a full in-game day takes.")]
    [SerializeField] private float _dayLengthInMinutes = 24f;
    [SerializeField] private Transform _sunLight;

    public float CurrentHour { get; private set; } = 8f; // Start at 8 AM
    public int CurrentDay { get; private set; } = 1;

    public event Action OnDayPassed;
    public event Action<float> OnHourChanged;

    private float _lastHourBroadcast = -1f;

    private void Update()
    {
        Tick(Time.deltaTime);
    }

    public void Tick(float deltaSeconds)
    {
        float timeScale = 24f / (_dayLengthInMinutes * 60f);
        CurrentHour += deltaSeconds * timeScale;

        if (CurrentHour >= 24f)
        {
            CurrentHour -= 24f;
            CurrentDay++;
            OnDayPassed?.Invoke();
        }

        int intHour = Mathf.FloorToInt(CurrentHour);
        if (intHour != _lastHourBroadcast)
        {
            _lastHourBroadcast = intHour;
            OnHourChanged?.Invoke(CurrentHour);
        }

        UpdateSunRotation();
    }

    private void UpdateSunRotation()
    {
        if (_sunLight == null) return;
        
        // 6 AM = 0 degrees (sunrise), 12 PM = 90 degrees (noon), 6 PM = 180 degrees (sunset)
        float sunAngle = (CurrentHour - 6f) / 24f * 360f;
        _sunLight.rotation = Quaternion.Euler(sunAngle, 0f, 0f);
    }

    public void SetTime(float newHour) => CurrentHour = Mathf.Repeat(newHour, 24f);
}

Best Practices

• ✅ Separate the time counting logic from the visual rendering updates.
• ✅ Use events (OnHourChanged, OnDayPassed) so other systems can react without polling.
• ✅ Scale sun intensity and ambient light based on time of day (using AnimationCurves).
• ❌ NEVER use physics or collision to detect time of day; just track the float value.
• ❌ NEVER bake lighting in a scene meant for a dynamic Day/Night cycle.

Related Skills

• @save-load-serialization — Save the CurrentDay and CurrentHour.
• @event-bus-system — Broadcast time changes globally without tight coupling.