I have 2 drop downs in HTML both representing months. So I want a validation like following.

If I select the first drop down month as April, then the next drop-down menu should start from the month April. If the first one is changed to June then the second should change to June.

<div id=head2 style=width:15%;float:right;margin-left:5px;>

    <select id='gMonth2' onchange=show_month()>

    <option value=''>--Select Month--</option>

    <option selected value='1'>Janaury</option>

    <option value='2'>February</option>

    <option value='3'>March</option>

    <option value='4'>April</option>

    <option value='5'>May</option>

    <option value='6'>June</option>

    <option value='7'>July</option>

    <option value='8'>August</option>

    <option value='9'>September</option>

    <option value='10'>October</option>

    <option value='11'>November</option>

    <option value='12'>December</option>

    </select> 

    </div>



<div id=head1 style=width:15%;float:right;margin-left:5px;>

        <select id='gMonth1'>

    <option value=''>--Select Month--</option>

    <option selected value='1'>Janaury</option>

    <option value='2'>February</option>

    <option value='3'>March</option>

    <option value='4'>April</option>

    <option value='5'>May</option>

    <option value='6'>June</option>

    <option value='7'>July</option>

    <option value='8'>August</option>

    <option value='9'>September</option>

    <option value='10'>October</option>

    <option value='11'>November</option>

    <option value='12'>December</option>

    </select> 

    </div> 

The function will be:

function show_month()

{

//

}

How should I write that function?