diff --git a/src/Resend/DateTimeOrHuman.cs b/src/Resend/DateTimeOrHuman.cs
index a5a17ca..550ed57 100644
--- a/src/Resend/DateTimeOrHuman.cs
+++ b/src/Resend/DateTimeOrHuman.cs
@@ -50,7 +50,7 @@ public override string ToString()
     public int CompareTo( DateTimeOrHuman other )
     {
         if ( this.IsMoment != other.IsMoment )
-            throw new NotSupportedException( "Cannot compare DateTimeOrHuman values of different kind" );
+            throw new NotSupportedException( "DH001: Cannot compare DateTimeOrHuman values of different kind" );
 
         if ( this.IsMoment == true )
             return this.Moment!.Value.CompareTo( other.Moment!.Value );
@@ -65,7 +65,7 @@ public static implicit operator DateTime( DateTimeOrHuman value )
         if ( value.IsMoment == true )
             return value.Moment!.Value;
 
-        throw new InvalidOperationException( "Value is not a DateTime" );
+        throw new InvalidOperationException( "DH002: Value is not a DateTime" );
     }
 
 
diff --git a/tests/Resend.Tests/DateTimeOrHumanTests.cs b/tests/Resend.Tests/DateTimeOrHumanTests.cs
index 8002d72..e7fe01f 100644
--- a/tests/Resend.Tests/DateTimeOrHumanTests.cs
+++ b/tests/Resend.Tests/DateTimeOrHumanTests.cs
@@ -15,6 +15,103 @@ public class WrapperClass
     }
 
 
+    /// <summary />
+    [Fact]
+    public void DateTimeToString()
+    {
+        var dt = DateTime.UtcNow;
+        DateTimeOrHuman src = dt;
+
+        Assert.Equal( dt.ToString(), src.ToString() );
+    }
+
+
+    /// <summary />
+    [Fact]
+    public void DateTimeCast()
+    {
+        var dt = DateTime.UtcNow;
+        DateTimeOrHuman src = dt;
+        DateTime tgt = src;
+
+        Assert.Equal( dt, tgt );
+    }
+
+
+    /// <summary />
+    [Fact]
+    public void DateTimeCastFromHuman()
+    {
+        DateTimeOrHuman src = "in 5 mins";
+
+        Action act = () =>
+        {
+            DateTime tgt = src;
+        };
+
+        var ex = Assert.Throws<InvalidOperationException>( act );
+        Assert.NotNull( ex.Message );
+        Assert.StartsWith( "DH002:", ex.Message );
+    }
+
+
+    /// <summary />
+    [Fact]
+    public void DateTimeComparable()
+    {
+        var dt1 = DateTime.UtcNow;
+        var dt2 = DateTime.UtcNow.AddDays( 1 );
+
+        DateTimeOrHuman src1 = dt1;
+        DateTimeOrHuman src2 = dt2;
+
+        Assert.Equal( dt1.CompareTo( dt2 ), src1.CompareTo( src2 ) );
+    }
+
+
+    /// <summary />
+    [Fact]
+    public void HumanToString()
+    {
+        var str = "in 5 mins";
+        DateTimeOrHuman v = str;
+
+        Assert.Equal( str, v.ToString() );
+    }
+
+
+    /// <summary />
+    [Fact]
+    public void HumanComparable()
+    {
+        var str1 = "in 5 mins";
+        var str2 = "tomorrow 5:00 am";
+
+        DateTimeOrHuman v1 = str1;
+        DateTimeOrHuman v2 = str2;
+
+        Assert.Equal( str1.CompareTo( str2 ), v1.CompareTo( v2 ) );
+    }
+
+
+    /// <summary />
+    [Fact]
+    public void MixedComparable()
+    {
+        var str = "in 5 mins";
+        var dt = DateTime.UtcNow;
+
+        DateTimeOrHuman v1 = str;
+        DateTimeOrHuman v2 = dt;
+
+        Action act = () => v1.CompareTo( v2 );
+
+        var ex = Assert.Throws<NotSupportedException>( act );
+        Assert.NotNull( ex.Message );
+        Assert.StartsWith( "DH001:", ex.Message );
+    }
+
+
     /// <summary />
     [Fact]
     public void PropertyNotNull()