When an Event is triggered by a user in IE, it is set to the window.event object. The only way to see what triggered the event is by accessing the window.
This is how I solved a simlar issue.
Wrote an onSelect() handler for the datepicker.
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In that function, called __doPostBack('textboxcontrolid','').
This triggered a partial postback for the textbox to the server, which called the validators in turn.