Se lo script non funziona posizionato ovunque nella pagina e' forse perche' gli altri script utilizzano altrove variabili che hanno lo stesso nome di quelle usate in questo... dovresti verificare.

Resta scontato che comunque hai la necessita' di richiamare le funzioni perche' agiscano... nell'esempio originale questo viene fatto utilizzando
<BODY onload="impostaDataCorrente()">

ma se non puoi mantenere la stessa chiamata devi posizionare lo script alla fine della pagina (subito prima del tag </body>) ed aggiungere la linea
impostaDataCorrente()
prima di </script>

Passiamo agli altri problemi (ti posto tutta la pagina)

codice:

<HTML> 
<HEAD> 
</HEAD> 
<BODY> 
<form name="sched"> 
 


 Arrivo: 
<select name="gg1"> 
<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">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="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> 
<select name="mm1"> 
<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">09</option> 
<option value="10">10</option> 
<option value="11">11</option> 
<option value="12">12</option> 
</select> 
<select name="aa1"> 
<option value="2000">2000</option> 
<option value="2001">2001</option> 
<option value="2002">2002</option> 
<option value="2003">2003</option> 
<option value="2004">2004</option> 
</select> 
</p> 
 

 


Orario di arrivo: 
<select name="orario"> 
<option value="01:00">01:00</option> 
<option value="02:00">02:00</option> 
<option value="03:00">03:00</option> 
<option value="04:00">04:00</option> 
<option value="05:00">05:00</option> 
<option value="06:00">06:00</option> 
<option value="07:00">07:00</option> 
<option value="08:00">08:00</option> 
<option value="09:00">09:00</option> 
<option value="10:00">10:00</option> 
<option value="11:00">11:00</option> 
<option value="12:00">12:00</option> 
<option value="13:00">13:00</option> 
<option value="14:00">14:00</option> 
<option value="15:00">15:00</option> 
<option value="16:00">16:00</option> 
<option value="17:00">17:00</option> 
<option value="18:00">18:00</option> 
<option value="19:00">19:00</option> 
<option value="21:00">21:00</option> 
<option value="22:00">22:00</option> 
<option value="23:00">23:00</option> 
<option value="24:00">24:00</option> 
</select> 
</p> 
 



 


 Partenza: 
<select name="dd2"> 
<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">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="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> 
<select name="mm2"> 
<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">09</option> 
<option value="10">10</option> 
<option value="11">11</option> 
<option value="12">12</option> 
</select> 
<select name="aa2"> 
<option value="2000">2000</option> 
<option value="2001">2001</option> 
<option value="2002">2002</option> 
<option value="2003">2003</option> 
<option value="2004">2004</option> 
</select> 
</p> 
 
</form> 
<script language="javascript"> 
function ImpostaSelect(select,xvalue){ 
aa=select.options 
for(i=0; i<aa.length; i++) 
if(aa[i].value==xvalue) 
aa[i].selected=true 
} 

function impostaDataCorrente(){ 
var today = new Date(); 
yyyy = today.getFullYear(); 
mm = today.getMonth()+1; 
//mmm = today.getMonth()+1; 
//ggg = today.getDate()+1; 
gg = today.getDate(); 
ora = String(100+today.getHours()).substr(1,2)+":00"; 
ImpostaSelect(document.sched.orario,ora); 
ImpostaSelect(document.sched.gg1,gg); 
ImpostaSelect(document.sched.mm1,mm); 
ImpostaSelect(document.sched.aa1,yyyy); 

var domani = new Date(); 
domani.setDate(domani.getDate()+1)
yyyy = domani.getFullYear(); 
mm = domani.getMonth()+1; 
gg = domani.getDate(); 
ImpostaSelect(document.sched.dd2,gg); 
ImpostaSelect(document.sched.mm2,mm); 
ImpostaSelect(document.sched.aa2,yyyy); 

} 
impostaDataCorrente()
</script> 
</BODY> 
</HTML>
ciao