日期控件如何默认为空

xiaoerp

请教一个问题，日期控件为Edit Box，但是总是有个默认值为当天， 现在想把这个值清空，默认为空，应该怎么设置？试过html控件的方式，好像没有用。
<script>
function subtractDay ()
{
var strDate ='';
return strDate;
}
pickerControlReceiptDateFrom.clearValue( );
</script>

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xiaoerp

//值提示去掉虚线和参数名

<script language="javascript">
       var form = getFormWarpRequest();
       var dropDown  = form._oLstChoices_updownlist;//_updownlist为值提示组件名
       dropDown.remove(1);
       dropDown.remove(0);
       dropDown.removeAttribute("hasLabel");  
</script>


//date prompt默认为当日
//昨日
<script>
function subtractDay ()
{
var dtToday = new Date();
var dtYesterday = new Date( dtToday - 86400000 );
var strYesterday = [dtYesterday.getUTCFullYear(), dtYesterday.getMonth()+1, dtYesterday.getDate()].join("-");
return strYesterday;
}
pickerControlPDate.setValue( subtractDay() );//PDate为date prompt组件名称（日期名称要与其一直）
</script>

//本周第一天
<script>
function   showWeekFirstDay()   
  {   
  var   Nowdate=new Date();   
  var   WeekFirstDay=new Date(Nowdate-(Nowdate.getDay()-1)*86400000);   
  var   strWeekFirstDay = [WeekFirstDay.getUTCFullYear(), WeekFirstDay.getMonth()+1, WeekFirstDay.getDate()].join("-");
  return   strWeekFirstDay;   
  }
pickerControlPDate.setValue( showWeekFirstDay() );
</script>

//本月第一天
<script>
function   showMonthFirstDay()   
  {   
  var   Nowdate=new Date();   
  var   MonthFirstDay=new Date(Nowdate-(Nowdate.getDay()-1)*86400000);   
  var   strMonthFirstDay = [MonthFirstDay.getUTCFullYear(), MonthFirstDay.getMonth()+1, MonthFirstDay.getDate()].join("-");
  return   strMonthFirstDay;   
  }
pickerControlPDate.setValue( showMonthFirstDay() );
</script>

//本月最后一天
<script>
function ShowMonthLastDay()   
  {   
  var   Nowdate=new Date();   
  var   MonthNextFirstDay=new   Date(Nowdate.getYear(),Nowdate.getMonth()+1,1);   
  var   MonthLastDay=new  Date(MonthNextFirstDay-86400000);   
  var   strMonthLastDay = [MonthLastDay.getUTCFullYear(), MonthLastDay.getMonth()+1, MonthLastDay.getDate()].join("-");
  return strMonthLastDay;
  }
pickerControlPDate.setValue( ShowMonthLastDay() );
</script>