PausableTimer.cs

using UnityEngine;

public class PausableTimer {
    private float _timerStart = -1;
    private float _timerTime;

    public PausableTimer(float initialTimerTime = 0) {
        _timerTime = initialTimerTime;
    }

    public float TimerTime {
        get {
            if (_timerStart != -1) {
                return _timerTime + Time.time - _timerStart;
            }
            return _timerTime;
        }
        set { _timerTime = value; }
    }

    public void StartTimer() {
        if (_timerStart == -1) {
            _timerStart = Time.time;
        }
    }

    public void PauseTimer() {
        if (_timerStart != -1) {
            _timerTime += Time.time - _timerStart;
            _timerStart = -1;
        }
    }

    public void ResetTimer() {
        PauseTimer();
        _timerTime = 0;
    }
}