diff --git a/DiscordChatExporter.Cli.Tests/DiscordChatExporter.Cli.Tests.csproj b/DiscordChatExporter.Cli.Tests/DiscordChatExporter.Cli.Tests.csproj
index 3e8b7a53..cd60c7b5 100644
--- a/DiscordChatExporter.Cli.Tests/DiscordChatExporter.Cli.Tests.csproj
+++ b/DiscordChatExporter.Cli.Tests/DiscordChatExporter.Cli.Tests.csproj
@@ -20,6 +20,7 @@
     <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="7.0.0" />
     <PackageReference Include="Microsoft.Extensions.Configuration.UserSecrets" Version="7.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.1" />
+    <PackageReference Include="ReflectionMagic" Version="4.1.0" />
     <PackageReference Include="System.Reactive" Version="5.0.0" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" PrivateAssets="all" />
diff --git a/DiscordChatExporter.Cli.Tests/Specs/HtmlMarkdownSpecs.cs b/DiscordChatExporter.Cli.Tests/Specs/HtmlMarkdownSpecs.cs
index e4436729..e889db33 100644
--- a/DiscordChatExporter.Cli.Tests/Specs/HtmlMarkdownSpecs.cs
+++ b/DiscordChatExporter.Cli.Tests/Specs/HtmlMarkdownSpecs.cs
@@ -1,6 +1,8 @@
-using System.Threading.Tasks;
+using System;
+using System.Threading.Tasks;
 using AngleSharp.Dom;
 using DiscordChatExporter.Cli.Tests.Infra;
+using DiscordChatExporter.Cli.Tests.Utils;
 using DiscordChatExporter.Core.Discord;
 using FluentAssertions;
 using Xunit;
@@ -12,125 +14,215 @@ public class HtmlMarkdownSpecs
     [Fact]
     public async Task Message_with_a_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323136411078787")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Default timestamp: 12-Feb-23 03:36 PM");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323136411078787")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Default timestamp: 02/12/2023 3:36 PM");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_short_time_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323205268967596")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Short time timestamp: 3:36 PM");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323205268967596")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Short time timestamp: 3:36 PM");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_long_time_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323235342139483")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Long time timestamp: 3:36:12 PM");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323235342139483")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Long time timestamp: 3:36:12 PM");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_short_date_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323326727634984")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Short date timestamp: 02/12/2023");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323326727634984")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Short date timestamp: 02/12/2023");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_long_date_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323350731640863")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Long date timestamp: February 12, 2023");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323350731640863")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Long date timestamp: February 12, 2023");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_full_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323374379118593")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Full timestamp: February 12, 2023 3:36 PM");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323374379118593")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Full timestamp: February 12, 2023 3:36 PM");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_full_long_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323409095376947")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Full long timestamp: Sunday, February 12, 2023 3:36 PM");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323409095376947")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Full long timestamp: Sunday, February 12, 2023 3:36 PM");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_a_relative_timestamp_is_rendered_as_the_default_timestamp()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074323436853285004")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Relative timestamp: 12-Feb-23 03:36 PM");
-        message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074323436853285004")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Relative timestamp: 02/12/2023 3:36 PM");
+            message.InnerHtml.Should().Contain("Sunday, February 12, 2023 3:36 PM");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 
     [Fact]
     public async Task Message_with_an_invalid_timestamp_is_rendered_correctly()
     {
-        // Act
-        var message = await ExportWrapper.GetMessageAsHtmlAsync(
-            ChannelIds.MarkdownTestCases,
-            Snowflake.Parse("1074328534409019563")
-        );
+        // Date formatting code relies on the local time zone, so we need to set it to a fixed value
+        TimeZoneInfoEx.SetLocal(TimeSpan.FromHours(+2));
 
-        // Assert
-        message.Text().Should().Contain("Invalid timestamp: Invalid date");
+        try
+        {
+            // Act
+            var message = await ExportWrapper.GetMessageAsHtmlAsync(
+                ChannelIds.MarkdownTestCases,
+                Snowflake.Parse("1074328534409019563")
+            );
+
+            // Assert
+            message.Text().Should().Contain("Invalid timestamp: Invalid date");
+        }
+        finally
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
     }
 }
\ No newline at end of file
diff --git a/DiscordChatExporter.Cli.Tests/Utils/TimeZoneInfoEx.cs b/DiscordChatExporter.Cli.Tests/Utils/TimeZoneInfoEx.cs
new file mode 100644
index 00000000..87c28c2a
--- /dev/null
+++ b/DiscordChatExporter.Cli.Tests/Utils/TimeZoneInfoEx.cs
@@ -0,0 +1,14 @@
+using System;
+using ReflectionMagic;
+
+namespace DiscordChatExporter.Cli.Tests.Utils;
+
+internal static class TimeZoneInfoEx
+{
+    // https://stackoverflow.com/a/63700512/2205454
+    public static void SetLocal(TimeZoneInfo timeZone) =>
+        typeof(TimeZoneInfo).AsDynamicType().s_cachedData._localTimeZone = timeZone;
+
+    public static void SetLocal(TimeSpan offset) =>
+        SetLocal(TimeZoneInfo.CreateCustomTimeZone("test-tz", offset, "test-tz", "test-tz"));
+}
\ No newline at end of file