Button's OnClick event can't call OnChange function of another DropDownList

Hi expert,
I am trying to use Classic version of CalendarXP (www.calendarxp.net/). Its ok. But I am facing another problem.
You know Classic version of CalendarXP has three DropDownLists(for Day, Month and Year) and one Button. When you click on button, a calendar pop is gererated. If you select a date from calendar pop, day will be shown on Day dropdownlist, month will be shown on Month DropDownList and year will be shown on year DropDownList. Each dropdownlist has onChange="if(self.gfPop)gfPop.updateHidden(this)". This function updates hidden text box, in where date is contained.
Problem is:
Before using CalendarXP control, I used a function  calculate(frmED,(passissDate_day.value),(passissDate_mon.value),(passissDate_year.value)) OnChange event of passissDate_year DropDownList. When I click on passissDate_year DropDownList, this OnChange function works. When I click on its button, calculate function doesnt work, but gfPop.updateHidden(this) works.

Code:

<select name="passissDate_year" class="listBox100" id="passissDate_year" style="width:60px;" onChange="calculate(frmED,(passissDate_day.value),(passissDate_mon.value),(passissDate_year.value))">
                  <script>document.write('<option value="-">year-');for(var i=1980;i<=2030;i++) {document.write('<option value="'+i+'">'+i)}</script>
                </select>

<INPUT name="popcal4" width="10" onclick="if(this.blur)this.blur();var fm=this.form;if(self.gfPop){gfPop.fStartPop(fm.passissDate,fm.passexpDate, this);calculate(frmED,(passissDate_day.value),(passissDate_mon.value),(passissDate_year.value));}" type="button" value="..." class="vbtn">


Regards,
Raju