alter the table to add a new field to house the new value.
Then make a small php script to do it for you.
//connect, blah blah, select db, blah
$sql = "SELECT unique_id,
date_field
FROM table";
$res = mysql_query($sql);
while($row=mysql_fetch_array($res)) {
mysql_query("UPDATE table
SET new_field = '".strtotime($row['date_field'])."'
WHERE unique_id = '".$row['unique_id']."'");
}
Then drop the existing date field. Alter the new field's name to what the old ield was named.