<script>
function replaceAmp() {
    var x = document.getElementsByClassName("errorMsg");
    if (x.length>0) {
        x[0].innerHTML = x[0].innerHTML.replace(/amp;/g, '');
    }
}
</script>

<apex:inputField value="{!opportunity.start_date__c}">
    <script type="text/javascript">
    (function(){
        replaceAmp();
    })();
    </script>
</apex:inputField>