Unity Timer

Timer.cs

/// <summary>
/// Simple manual timer class 
/// </summary>
[System.Serializable]
public class Timer
    {
    public float startTime = 0;
    public float endTime = 0;

    public bool IsFinished => currentTime >= endTime;

    [UnityEngine.SerializeField]
    private float currentTime;

    public Timer(float endTime)
        {
        this.endTime = endTime;
        }

    /// <summary>
    /// Update the timer value
    /// </summary>
    /// <param name="deltaTime"></param>
    public void UpdateTimer(float deltaTime)
        {
        if (currentTime > endTime)
            currentTime = startTime;
        else
            currentTime += deltaTime;
        }
    }

TimerFull.cs

using System;

/// <summary>
/// Manually updated timer class 
/// </summary>
[System.Serializable]
public class Timer
    {
    public float startTime = 0;
    public float endTime = 0;

    [UnityEngine.SerializeField]
    private float currentTime;
    public Action onFinish;

    public float CurrentTime => currentTime;

    public bool IsFinished => currentTime >= endTime;
    public bool IsStarting => currentTime <= startTime;

    public float CurrentTimeNormalized => currentTime / endTime;

    public Timer(float endTime = 1, float startTime = 0, float currentTime = 0)
        {
        this.startTime = startTime;
        this.endTime = endTime;
        this.currentTime = currentTime;
        }

    /// <summary>
    /// Update the timer value
    /// </summary>
    /// <param name="deltaTime"></param>
    public void UpdateTimer(float deltaTime)
        {
        if (currentTime >= endTime)
            currentTime = startTime;
        else
            currentTime += deltaTime;

        if (IsFinished)
            onFinish?.Invoke ();
        }

    /// <summary>
    /// Manually reset the timer to it's start time
    /// </summary>
    public void ResetTimer()
        {
        currentTime = 0;
        }

    /// <summary>
    /// Set the timer position within range of min and max (inclusive)
    /// </summary>
    /// <param name="value">New value for the timer</param>
    public void SetTimerPosition(float value)
        {
        currentTime = UnityEngine.Mathf.Clamp (value, startTime, endTime);
        }

    public void SetTimerPositionNormalized(float value)
        {
        currentTime = endTime * value;
        }

    public override String ToString()
        {
        return $"Timer: {currentTime}; {startTime} | {endTime}";
        }
    }