Hi,
It is not clear what happens. That position-move happens because you process ValueChange event, or it happens when you open drop-down calendar, or after drop-down calendar is closed. If the reason is processing client event, then your codes probably restructure layout of html and if location of datechooser depends on it, then it may move. If you have a simple aspx (not a large application filled with custom javascripts), then you may attach it (go to Options tab).