title: foolproof validation greaterthan error System.NotImplementedException
body: <p>im using foolproof validation for hours</p>
  
  <pre><code>[Required]
  [Range(typeof(TimeSpan), "00:00:00", "23:59:59")]
  public Nullable&lt;System.TimeSpan&gt; start { get; set; }
  [Required]
  [GreaterThan("HoraInicio")]
  public Nullable&lt;System.TimeSpan&gt; end { get; set; }
  </code></pre>
  
  <p>it validates the fields correctly but when i call submitchanges in the controller it throws me an error</p>
  
  <blockquote>
    <p>[NotImplementedException: No se puede implementar el método o la operación.]
       Foolproof.ModelAwareValidationAttribute.IsValid(Object value) +59
       System.ComponentModel.DataAnnotations.ValidationAttribute.IsValid(Object value, ValidationContext validationContext) +115
       System.ComponentModel.DataAnnotations.ValidationAttribute.GetValidationResult(Object value, ValidationContext validationContext) +29
       System.Data.Entity.Internal.Validation.ValidationAttributeValidator.Validate(EntityValidationContext entityValidationContext, InternalMemberEntry property) +198</p>
    
    <p>[DbUnexpectedValidationException: An unexpected exception was thrown during validation of 'end' when invoking Foolproof.GreaterThanAttribute.IsValid. See the inner exception for details.]</p>
  </blockquote>
  
  <p>happens the same with dates, what can i do? is there any other way to do this validation?</p>