Reset Cookies window.onload in Java Script

<Script language ="Javascript">
 function notNull()
 {
    if (document.LvDtlsFrm.calcDT.value == null || document.LvDtlsFrm.calcDT.value=="")
    alert("Please select Calculation Date:");
    else{
   
      document.LvDtlsFrm.submit();}
 }
var my_time = new Date();
function getCookie(c_name){     if (document.cookie.length>0){         c_start=document.cookie.indexOf(c_name + "=");         if (c_start!=-1){                  c_start=c_start + c_name.length+1;                  c_end=document.cookie.indexOf(";",c_start);                 if (c_end==-1) c_end=document.cookie.length;                 return unescape(document.cookie.substring(c_start,c_end));         }      }     return ""; } 
function setCookie(c_name,value,expiredays){     var exdate=new Date();     exdate.setDate(exdate.getDate()+expiredays);     document.cookie=c_name+ "=" +escape(value)+     ((expiredays==null) ? "" : ";expires="+exdate.toGMTString()); } 
function checkCookie(){      username=getCookie('username');     if(username!=null && username!="" && username == my_time ){    return false;     }else{ setCookie('username',my_time,365); document.LvDtlsFrm.submit();  } }
window.onload = checkCookie;
</Script>

use this notNull function also for button
<td align="right" nowrap><input type="button" name="LvbalDT" value="Submit" onclick="notNull()"></td>