How to turn off validation

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WPF

validation

MVVM

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Hello
I'm using Data Annotation in a Calburn Micro framework to validate my user input. User selects a date that needs to be 3 days in the future. Record is saved to db. 4 days later the record is opened again and validation kicks in on the selected date. What I need to do is turn off validation for my "RequiredStartDateTime" object on any old database records. I.E only validate when record is new. Can anyone advise on how to do this?

private DateTime? _RequiredStartDateTime;        
        [Required(ErrorMessageResourceType = typeof(Properties.Resources), ErrorMessageResourceName = "ErrorMessageNullDate")]
        [ProcessingDelay001(ErrorMessageResourceType = typeof(Properties.Resources), ErrorMessageResourceName = "ErrorMessageRequiredStartDateTime")]
        public DateTime? RequiredStartDateTime
        {
            get { return _RequiredStartDateTime; }
            set 
            { 
               _RequiredStartDateTime = value;
               NotifyOfPropertyChange(() => RequiredStartDateTime);
               _events.PublishOnUIThread(new IsValidNotifier(NoValidationErrors));
            }
        }