Gracias por tu tiempo David.
Requiero que si LA FACTURA NO HA SIDO PAGADA, el FORM envie la FECHA '0000-00-00'.
Aqui esta el FORM Completo:
Código PHP:
<form action="" id="formfact" onsubmit="if(document.forms.formfact.fact_pagada.value!=1){document.forms.formfact.fact_a1.options[0].value='0000';document.forms.formfact.fact_m1.options[0].value='00';document.forms.formfact.fact_d1.options[0].value='00';alert(document.forms.formfact.fact_d1.value);return false;}"><input type="hidden" name="accion" value="fact" /><input type="hidden" name="fact_user" value="4187" /><table id="factura" border="1" cellpadding="5" class="c_tm"><tr><td><select name="fact_pagada" class="fwb sel_cm" onchange="if(this.value!=1)document.getElementById('fact_fecha_pago').style.display='none';else document.getElementById('fact_fecha_pago').style.display='block';"><option value="0">Aún no pagada</option><option value="1">Fecha de pago</option></select></td><td class="tn" id="fact_fecha_pago" style="display:none;"><select name="fact_m1" class="sel_c">
<option value="01" selected="selected">Enero</option>
<option value="02">Febrero</option>
<option value="03">Marzo</option>
<option value="04">Abril</option>
<option value="05">Mayo</option>
<option value="06">Junio</option>
<option value="07">Julio</option>
<option value="08">Agosto</option>
<option value="09">Septiembre</option>
<option value="10">Octubre</option>
<option value="11">Noviembre</option>
<option value="12">Diciembre</option>
</select><select name="fact_d1" class="sel_c">
<option value="01">01</option>
<option value="02">02</option>
<option value="03">03</option>
<option value="04">04</option>
<option value="05">05</option>
<option value="06">06</option>
<option value="07">07</option>
<option value="08">08</option>
<option value="09" selected="selected">09</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
<option value="16">16</option>
<option value="17">17</option>
<option value="18">18</option>
<option value="19">19</option>
<option value="20">20</option>
<option value="21">21</option>
<option value="22">22</option>
<option value="23">23</option>
<option value="24">24</option>
<option value="25">25</option>
<option value="26">26</option>
<option value="27">27</option>
<option value="28">28</option>
<option value="29">29</option>
<option value="30">30</option>
<option value="31">31</option>
</select> de <select name="fact_a1" class="sel_c">
<option value="2012">2012</option>
<option value="2013" selected="selected">2013</option>
</select></td></tr><tr><td class="tn tc" colspan="2"><input type="submit" value="Guardar Factura" class="b" /></td></tr></table></form>