jQuery UI: DatePicker, select only today's date through past

I'm using the datePicker in the jQuery UI core. I need a date picker that can only pick dates through the past all the way to today.

Is there an easy way to do this? -- Note I'm using the UI core, not the DatePicker Plugin.

My jQuery call:

$(function() {
     $(".datepicker").datepicker();
 });
 

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