The textbox validation can be done using the built-in validation in Silverlight 3.0 (if you are using 3.0)
The following link should help you to validate both the textbox and date. It may not be exactly what you want but after understanding it you can modify it or may be improvise it for your use.
http://silverlighttips.com/?tag=/validation[
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Let me know if this helps you
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AJ