Salesforce Visualforce Page - Disable previous date in calendar

VF Page-

<apex:page showHeader="false" controller="Test4">
<head>
<apex:stylesheet value="https://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css" />
<apex:includeScript value="https://code.jquery.com/jquery-1.9.1.js" />
<apex:includeScript value="https://code.jquery.com/ui/1.10.3/jquery-ui.js" />
</head>

<script>
var today = new Date();
$("#datepicker").datepicker({
changeMonth: true,
changeYear: true,
minDate: today
});
</script>
<apex:form >
<apex:pageBlock >
<apex:selectList value="{!selectId}" size="1">
<apex:selectOptions value="{!displayOptions}"></apex:selectOptions>
<apex:actionSupport event="onchange" action="{!changeDateRange}" reRender="calender"/>
</apex:selectList>
<input type="text" id="datepicker"/>
</apex:pageBlock>
</apex:form>
</apex:page>

Controller-

public class Test4 {

public List<selectoption> displayOptions { get; set; }
public String selectId { get; set; }
public Date myDate {get;set;}

public PageReference changeDateRange() {
myDate = Date.today();
if(selectId.equals('10')){
myDate = myDate.addDays(10);
} else if(selectId.equals('20')){
myDate = myDate.addDays(20);
}
return null;
}

public test4()
{
myDate = Date.today();
displayOptions =new List<selectoption>();
displayOptions.add(new selectoption('none','-select-'));
displayOptions.add(new selectoption('10','Next 10 Days'));
displayOptions.add(new selectoption('20','Next 20 Days'));
}

}

I want to disable previous date in calendar but its not working.