$('#datepicker').datepicker("setDate", "+3");
Working example: http://jsfiddle.net/EryYr/1/
Quote from the documentation:
SETDATE .datepicker( "setDate" , date ) ... The new date can be a Date object or a string in the current date format (for example, '01 / 26/2009 '), a few days from today (for example, +7) or strings of values ββand periods ('y' for years, 'm' for months, 'w' for weeks, 'd' for several days, for example. '+ 1m + 7d'), or null to clear the selected date .
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