I found my own solution. to set date for a c1WebdateEdit control, use the following code
var strdate=dtsheet.split("-");
var sheetdate =new Date(strdate[2] +"/"+ strdate[1] +"/"+ strdate[0]);
<%=dtpstartdate.ClientObjectID%>.set_Value(sheetdate);
to get the date,
var dt=<%=dtpstartdate.ClientObjectID%>.get_Value();