try onselect handler
there is no onselect event for for datetime (tried it however), also tried change and blur neither seems to invoked
datetime has a property minDate & maxdate. You can utilize that & set value dynamically.
Example For your use case create a listener node under your 'datetime' xtype node and create a property 'beforerender" (which is basically an event) of type string
and add the code like  . WIth this if you try to select past date the error is thrown. Hope this helps.
var minDate = new Date();
//setting mindate to be selected