Validation Application Block and PropertyProxyValidator
I had some trouble validating date input using VAB and the PropertyProxyValidator. As I would think would be the norm, for a date that I have specific requirements on, I'll setup a NotNullValidator, along with a DateRangeValidator to make sure the date fits within a certain range. But I was getting errors because the PropertyProxyValidator was tied to a TextBox control, which means that the underlying value is a string. Because it's a string, it couldn't be validated against.
The ValueConvert event fires whenever a server-side validation occurs (which is the mode that it works in), allowing you to convert the value to an alternative format. This allowed me to convert the date from a string to a DateTime reference, and this solved the problem.