Adding more tests to cover the ContentService and refactoring.

src/Umbraco.Core/Persistence/Querying/ExpressionHelper.cs

@@ -479,6 +479,9 @@ namespace Umbraco.Core.Persistence.Querying
             if (fieldType == typeof(decimal))
                 return ((decimal)value).ToString(CultureInfo.InvariantCulture);
 
+            if(fieldType == typeof(DateTime))
+                return "'" + EscapeParam(((DateTime)value).ToString(CultureInfo.InvariantCulture)) + "'";
+
             return ShouldQuoteValue(fieldType)
                     ? "'" + EscapeParam(value) + "'"
                     : value.ToString();