The ClientID of the date picker in this case is more than just its server id and a suffix to indicate the input. Since the control is in a grid hierarchy there are several other INaming containers above it, so its ID is different. It can be accessed in when the edit item is created, as shown in this help article: http://www.telerik.com/help/aspnet-ajax/grid-reference-controls-in-edit-form.html
. For your convenience I am attaching a simple page that shows this in action. Note that I have chosen the input wrapper as a target for the tooltip, yet you can inspect the rendered HTML and choose the suffix for the button or the entire datepicker control.
All the best,
the Telerik team
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