Unity Timer Countdown system for upgrading items over a specific time span

UpgradeTimerManager.cs

using System.Collections.Generic;
using System;
using System.Linq;
using UnityEngine;

[Serializable]
public class UpgradeTimerManager {

    public string id { get; private set; } 

    [SerializeField] List<Item> items = new();
    
    static string getKey(string id) => $"UpgradeTimerManager_{id}";
    
    UpgradeTimerManager(string id) => this.id = id;

    public static UpgradeTimerManager Load(string id) {
        var key = getKey( id );
        if (PlayerPrefs.HasKey( key )) {
            var manager = JsonUtility.FromJson<UpgradeTimerManager>( PlayerPrefs.GetString( key ) );
            manager.id = id;
            return manager;
        }
        else {
            return new UpgradeTimerManager( id );
        }

    }


    public void Save() {
        var key = getKey( id );
        if (items.Count == 0) PlayerPrefs.DeleteKey( key );
        else {
            var value = JsonUtility.ToJson( this, false );
            PlayerPrefs.SetString( key, value );
            PlayerPrefs.Save();
        }
    }

    public void Clear() => items = new List<Item>();

    public bool IsItemUpgrading(string id) {
        var item = items.Find( item => item.id == id );
        if (item == null) return false;
        var finished = item.endTime < getTime();
        if (finished) DeleteItem( id ); // lazy remove
        return !finished;
    }


    public void StartNewUpgrade(string id, TimeSpan duration) {
        if (IsItemUpgrading( id )) return;
        items.Add( new Item { id = id, endTimeTicks = ( getTime() + duration ).Ticks } );
    }

    public TimeSpan GetRemainingTimeFor(string id) {
        if (!IsItemUpgrading( id )) throw new Exception( "Item is not upgrading" );
        return new DateTime( items.First( item1 => item1.id == id ).endTimeTicks ) - getTime();
    }

    public void DeleteItem(string id) => items.RemoveAll( item => item.id == id );

    DateTime getTime() => new(DateTime.UtcNow.Ticks);

    [Serializable]
    class Item {
        public string id;
        public long endTimeTicks;
        public DateTime endTime => new DateTime( endTimeTicks );
    }
}

UpgradeTimerManager_Test.cs

using System;
using System.Collections;
using UnityEngine;

public class UpgradeTimerManager_Test {

    [UnityEngine.TestTools.UnityTest]
    public IEnumerator UpgradeTimerManagerTestWithEnumeratorPasses() {
        Debug.Log( "test started." );

        var upgradeTimerManager = UpgradeTimerManager.Load( "test" );
        upgradeTimerManager.Clear();
        upgradeTimerManager.Save();
        upgradeTimerManager.StartNewUpgrade( "A", TimeSpan.FromSeconds( 2 ) );
        upgradeTimerManager.StartNewUpgrade( "B", TimeSpan.FromSeconds( 3 ) );
        upgradeTimerManager.StartNewUpgrade( "C", TimeSpan.FromSeconds( 4 ) );
        Debug.Log( $"remaining time for A: {upgradeTimerManager.GetRemainingTimeFor( "A" )}" );
        Debug.Log( $"remaining time for B: {upgradeTimerManager.GetRemainingTimeFor( "B" )}" );
        Debug.Log( $"remaining time for C: {upgradeTimerManager.GetRemainingTimeFor( "C" )}" );
        upgradeTimerManager.Save();

        yield return new WaitForSecondsRealtime( 1 );

        // test it's save functionality
        upgradeTimerManager = UpgradeTimerManager.Load( "test" ); // reload
        Debug.Log($"contains 1: {upgradeTimerManager.IsItemUpgrading( "A" )}: {upgradeTimerManager.GetRemainingTimeFor( "A" )}" );
        Debug.Log($"contains 2: {upgradeTimerManager.IsItemUpgrading( "B" )}: {upgradeTimerManager.GetRemainingTimeFor( "B" )}" );
        Debug.Log($"contains 3: {upgradeTimerManager.IsItemUpgrading( "C" )}: {upgradeTimerManager.GetRemainingTimeFor( "C" )}" );

        yield return new WaitForSecondsRealtime( 1 );
        Debug.Log( $"contains 1: {upgradeTimerManager.IsItemUpgrading( "A" )}" );
        Debug.Log($"contains 2: {upgradeTimerManager.IsItemUpgrading( "B" )}: {upgradeTimerManager.GetRemainingTimeFor( "B" )}" );
        Debug.Log($"contains 3: {upgradeTimerManager.IsItemUpgrading( "C" )}: {upgradeTimerManager.GetRemainingTimeFor( "C" )}" );

        yield return new WaitForSecondsRealtime( 1 );
        Debug.Log( $"contains 1: {upgradeTimerManager.IsItemUpgrading( "A" )}" );
        Debug.Log( $"contains 2: {upgradeTimerManager.IsItemUpgrading( "B" )}" );
        Debug.Log($"contains 3: {upgradeTimerManager.IsItemUpgrading( "C" )}: {upgradeTimerManager.GetRemainingTimeFor( "C" )}" );

        yield return new WaitForSecondsRealtime( 1 );
        Debug.Log( $"contains 1: {upgradeTimerManager.IsItemUpgrading( "A" )}" );
        Debug.Log( $"contains 2: {upgradeTimerManager.IsItemUpgrading( "B" )}" );
        Debug.Log( $"contains 3: {upgradeTimerManager.IsItemUpgrading( "C" )}" );

        upgradeTimerManager.Clear();
        upgradeTimerManager.Save();

        Debug.Log( "test finished." );
    }
}

test output:

test started.
remaining time for A: 00:00:02
remaining time for B: 00:00:03
remaining time for C: 00:00:04
--[1 second later]
contains 1: True: 00:00:00.9943653
contains 2: True: 00:00:01.9943653
contains 3: True: 00:00:02.9943653
--[1 second later]
contains 1: False
contains 2: True: 00:00:00.9897110
contains 3: True: 00:00:01.9897110
--[1 second later]
contains 1: False
contains 2: False
contains 3: True: 00:00:00.9851640
--[1 second later]
contains 1: False
contains 2: False
contains 3: False
test finished.

sample saved json from the test:

{
  "items": [
    {
      "id": "A",
      "endTimeTicks": 638196595733375993
    },
    {
      "id": "B",
      "endTimeTicks": 638196595743375993
    },
    {
      "id": "C",
      "endTimeTicks": 638196595753375993
    }
  ]
}