I have the jQuery date picker setup and working but would like help with setting the minDate and maxDate options. My current code is below (without these options). How can I set the minDate as 3 months before the defaultDate, and maxDate as 28days after the defaultDate?
var expdisp = $("#expdisp").attr("value");
$("#expirydate" ).datepicker({
    showOn: "button",
    buttonImage: "images/calendar.gif",
    buttonImageOnly: true,
    dateFormat: "dd/mm/yy",
    defaultDate: expdisp,
    showOtherMonths: true,
    selectOtherMonths: true,
    changeMonth: true,
    changeYear: true,
});
                $(function() {
    $( "#datepicker" ).datepicker({ 
        changeYear: true,
        minDate: '-3M',
        maxDate: '+28D',
    });
});
JSFiddle Demo
UPDATE
You can calculate tour max and min valid dates from the default date, then assign it to the date picker.
var expdisp = $("#expdisp").attr("value");
$("#expirydate" ).datepicker({
    showOn: "button",
    buttonImage: "images/calendar.gif",
    buttonImageOnly: true,
    dateFormat: "dd/mm/yy",
    defaultDate: expdisp,
    showOtherMonths: true,
    selectOtherMonths: true,
    changeMonth: true,
    changeYear: true,
    minDate: '-3M',
    maxDate: '+28D',
});
Update Demo
maxDate :- Sets the maximum date that can be selected. Accepts a date object or a relative number. For example: +7, or a string such as +6m.
minDate :- Sets the minimum date that can be selected. Accepts a number, date object, or string.
$(document).ready(function() {
  $("#date").datepicker({
      minDate: -3,
      maxDate: "1w"
  });
});
Refer:-set Minimum and maximum date to jquery datepicker
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