Remove jcalendar
The jcalendar widget is almost gone from core - it's still used from civirules - search for CRM/common/jcalendar.tpl
It's generally pretty easy - at the tpl level you just need to render the field html like for any other field. You need to remove any weird date handling like CRM_Utils_Date::processDate because the js standardises the dates with datepicker
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