Hi David,
Sorry for the delay in getting back to you.
We have created a sample based on your requirement and you can download the sample from the below link.
In this sample, we have created the custom property Date to validate the DateTime in DateTimeEdit control through ValidatesOnDataErrors in binding and we have overriden the Validation border while DateTimeEdit control get focused by custom style which collapsed the FocusBorder and gives the validation style as you have required.
Please let us know if you have any queries.
Regards,
Karuppasamy P
DateTimeEditDemo_f30c6b54.zip