how to get date from one textbox into another which is in item template of gridview

Hi In a grid view i have two textboxes in item template. and txtIssueDate and txtExpiryDate. When a date is entered in txtIssueDate  2 years should be added and need to display the value in txtExpiryDate. How i can achieve this.
<table>
        <tr>
            <td align="left">
                <div style="height: 275px; overflow: auto"> Go to the complete details ...