diff --git a/tests/AppMotor.Core.Tests/Tests/Utils/DateTimeUtcTests.cs b/tests/AppMotor.Core.Tests/Tests/Utils/DateTimeUtcTests.cs
index c1306d5a..c2831ec2 100644
--- a/tests/AppMotor.Core.Tests/Tests/Utils/DateTimeUtcTests.cs
+++ b/tests/AppMotor.Core.Tests/Tests/Utils/DateTimeUtcTests.cs
@@ -4,6 +4,7 @@
 using System.Globalization;
 using System.Text.Json;
 
+using AppMotor.Core.Extensions;
 using AppMotor.Core.Utils;
 using AppMotor.TestCore;
 using AppMotor.TestCore.Shouldly;
@@ -278,6 +279,7 @@ public void Test_ToString()
         dateTimeUtc.ToString().ShouldBe(now.ToString());
         dateTimeUtc.ToString(null).ShouldBe(now.ToString((string?)null));
         dateTimeUtc.ToString(null, null).ShouldBe(now.ToString(null, null));
+        dateTimeUtc.ToString("o").ShouldBe(now.ToStringIC("yyyy-MM-ddTHH:mm:ss.fffffffZ"));
     }
 
     [Fact]