Added FakeLocalTimeZone helper class.

The class is used to change the time zone of the tests upon initialization, and resets it back when disposed.

Author: AhmedZaki99

src/Todoist.Net.Tests/Helpers/FakeLocalTimeZone.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Reflection;
+
+namespace Todoist.Net.Tests.Helpers;
+
+/// <summary>
+/// A helper class that changes the local timezone to a fake timezone provided
+/// at initialization, and resets the original local timezone when disposed.
+/// </summary>
+/// <remarks>
+/// See this <see href="https://stackoverflow.com/questions/44413407/mock-the-country-timezone-you-are-running-unit-test-from">SO question</see> for more details.
+/// </remarks>
+public sealed class FakeLocalTimeZone : IDisposable
+{
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="FakeLocalTimeZone"/> class
+    /// and changes the local time zone to the given <paramref name="fakeTimeZoneInfo"/>
+    /// until it's disposed.
+    /// </summary>
+    /// <param name="fakeTimeZoneInfo">The time zone to set as local until disposal.</param>
+    public FakeLocalTimeZone(TimeZoneInfo fakeTimeZoneInfo)
+    {
+        var info = typeof(TimeZoneInfo).GetField("s_cachedData", BindingFlags.NonPublic | BindingFlags.Static);
+        var cachedData = info.GetValue(null);
+
+        var field = cachedData.GetType().GetField("_localTimeZone",
+            BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static | BindingFlags.Instance);
+
+        field.SetValue(cachedData, fakeTimeZoneInfo);
+    }
+
+    public void Dispose()
+    {
+        TimeZoneInfo.ClearCachedData();
+    }
+}

src/Todoist.Net.Tests/Helpers/FakeLocalTimeZoneTests.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Linq;
+
+using Todoist.Net.Tests.Extensions;
+
+using Xunit;
+
+namespace Todoist.Net.Tests.Helpers;
+
+[Trait(Constants.TraitName, Constants.UnitTraitValue)]
+public class FakeLocalTimeZoneTests
+{
+
+    [Fact]
+    public void FakeLocalTimeZone_ShouldChangeLocalTimeZoneWithinScope_AndResetItBackOutsideScope()
+    {
+        var actualTimeZoneInfo = TimeZoneInfo.Local;
+
+        var timeZoneCollection = TimeZoneInfo
+            .GetSystemTimeZones()
+            .Where(t => !actualTimeZoneInfo.Equals(t))
+            .ToArray();
+
+        var randomIndex = new Random().Next(timeZoneCollection.Length);
+        var fakeTimeZoneInfo = timeZoneCollection.ElementAt(randomIndex);
+
+
+        Assert.NotEqual(fakeTimeZoneInfo, actualTimeZoneInfo);
+
+        using (var fakeLocalTimeZone = new FakeLocalTimeZone(fakeTimeZoneInfo))
+        {
+            Assert.Equal(fakeTimeZoneInfo, TimeZoneInfo.Local);
+        }
+        Assert.Equal(actualTimeZoneInfo, TimeZoneInfo.Local);
+    }
+
+}

src/Todoist.Net.Tests/Models/DueDateTests.cs

@@ -2,13 +2,35 @@
 
 using Todoist.Net.Models;
 using Todoist.Net.Tests.Extensions;
+using Todoist.Net.Tests.Helpers;
+
 using Xunit;
 
 namespace Todoist.Net.Tests.Models
 {
     [Trait(Constants.TraitName, Constants.UnitTraitValue)]
-    public class DueDateTests
+    public class DueDateTests : IDisposable
     {
+
+        private readonly FakeLocalTimeZone _fakeLocalTimeZone;
+
+        public DueDateTests()
+        {
+            var timeZoneCollection = System.TimeZoneInfo.GetSystemTimeZones();
+
+            var randomIndex = new Random().Next(timeZoneCollection.Count);
+            var fakeTimeZoneInfo = timeZoneCollection[randomIndex];
+
+            _fakeLocalTimeZone = new FakeLocalTimeZone(fakeTimeZoneInfo);
+        }
+
+        public void Dispose()
+        {
+            _fakeLocalTimeZone.Dispose();
+            GC.SuppressFinalize(this);
+        }
+
+
         [Fact]
         public void DateTimeAssignment_FullDayEvent_Success()
         {