Need help with this code

[valleyjim]

<SCRIPT  LANGUAGE="JavaScript">
var now = new Date();
var hr = now.getHours();
var min = now.getMinutes();

if (hr == 18) && (min == 0) // The time I want this to calculate
var tf = <!--outsidetemp--> // this is a tag generated by my weather station, it changes as temp. changes
var df = <!--outsidedewpt--> // this is a tag generated by my weather station, it changes as dewpoint changes
tc = 5/9* (tf - 32);
tc = Math.round((tc*10))/10;
dc = 5/9*(df - 32)
dc=Math.round((dc*10))/10;
m = tc- (tc - dc) /2 -3.5 
el = 9/5* (ml) +32 
el = Math.round((el*1))/1; // I want to keep this variable without it changing until the next time the time is 18
mh = tc + (tc - dc) /2 +5.7 
eh = 9/5 * (mh) + 32
eh = Math.round((eh*1))/1;  // I want to keep this variable without it changing until the next time the time is 18


document.write("Estimated Low Temperature " + el) //?
document.write("Estimated Low Temperature " + eh) //?
</SCRIPT>