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One contender is YUI Calendar http://developer.yahoo.com/yui/calendar/ which has a BSD license.
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The Interval Selection calendar is quite impressive. http://developer.yahoo.com/yui/examples/calendar/intervalcal.html
There appears to be no time support.
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Another contender is jQuery DatePicker http://jqueryui.com/demos/datepicker/ which has a MIT and LGPL license.
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Another option is CalendarDateSelect http://electronicholas.com/calendar which has a MIT license.
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