Based on CalendarExtender, you can set the format as "MM / dd / yyyy". After the user selects a date in the calendar, he will return, for example, 04/28/2009. In the selected date event, you can add the current time after the date is returned.
OnClientDateSelectionChanged = "dateselect"
function dateselect(ev)
{
var calendarBehavior1 = $find("Calendar1");
var d = calendarBehavior1._selectedDate;
var now = new Date();
calendarBehavior1.get_element().value = d.format("MM/dd/yyyy") + " "+now.format("HH:mm:ss")
}
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