I cannot seem to get validation to work correctly on a template datepicker column, any suggestions?

This is the property I am binding to:

[System.Runtime.Serialization.DataMemberAttribute(Name="End Date")]
public System.DateTime EndDate {
get {
return this.EndDateField;
}
set {
if ((this.EndDateField.Equals(value) != true)) {

if (value < StartDate)
throw new System.Exception("The end date must occur after the start date");

if(value.Month != StartDate.Month || value.Year != StartDate.Year)
throw new System.Exception("Occurrences may not span multiple months or years");

this.EndDateField = value;
this.RaisePropertyChanged("EndDate");
}
}
}

and this is the xaml for the column:

<igGrid:TemplateColumn HeaderText="End Date" Key="EndDate">
<igGrid:TemplateColumn.ItemTemplate>
<DataTemplate>
<TextBlock  Text="{Binding EndDate}"/>     
</DataTemplate>
</igGrid:TemplateColumn.ItemTemplate>
<igGrid:TemplateColumn.EditorTemplate>
<DataTemplate>
<basics:DatePicker SelectedDate="{Binding EndDate, Mode=TwoWay, ValidatesOnExceptions=True, NotifyOnValidationError=True, UpdateSourceTrigger=Explicit}" />   
</DataTemplate>
</igGrid:TemplateColumn.EditorTemplate>
</igGrid:TemplateColumn>