How to Set Maximumvalue Property on Rangevalidator

I have a "Date" type rangevalidator tied to a textbox control.  I want to set the maximumvalue and minimumvalue properties of the validator (not in code behind, but in the properties of the control in design mode).  I want the maximumvalue to be today's date (whatever day the user uses the form) and today's date minus 15 years.  I have tried the following:
DateTime.Now
DateTime.Now.AddYears(-15)
But the page throws the error:
The value 'DateTime.Now' of the MaximumValue property of 'RangeValidator1' cannot be converted to type 'Date'.
What should I put into the propeties to accomplish what I am after?  Thanks for any help.
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