I am trying to determine the start and end dates of daylight savings time (DST) according to the user's locale specified in their system. I am so close, but I am off by one hour. Here in the Florida, DST starts at Sun Mar 11 02:00:00 EST 2007, but my function returns Sun Mar 11 01:00:00 EST 2007. I commentted my code to describe how I am accomplishing the goal.
This code may not work in the southern hemisphere at this time since their seasons are reversed, but I will worry about that after I get this off-by-one bug resolved.
Thanks,
Mike
Code:
<head>
<script>
function displayDSTdates(){
var rightNow = new Date();
var dJanuaryFirst= new Date(rightNow.getFullYear(), 0, 1, 0, 0, 0, 0); //start at jauary first
var cTemp = dJanuaryFirst.toGMTString(); //convert to GMT string
var dJanuaryFirstGMT = new Date(cTemp.substring(0, cTemp.lastIndexOf(" ")-1)); //get GMT date object
var nStandardGMToffset = ((dJanuaryFirst - dJanuaryFirstGMT) / (1000 * 60 * 60)); // get GMT offset for January First
var bDSTstartDateFound = false;
var nHoursInYear = (365 * 24);// not accurate for leap year, but shouldn't matter since a DST start / end date never falls on the last day of the year.
for(var i=0;i<nHoursInYear;i++){// Loop through all 8760 hours in a year
var dSampleDate = new Date(dJanuaryFirst); // create new date object starting a Jan 1st
dSampleDate.setHours(i);// advance i hours
var cSampleTemp = dSampleDate.toGMTString();
var dSampleDateGMT = new Date(cSampleTemp.substring(0, cSampleTemp.lastIndexOf(" ")-1));
var nCurrentGMToffset = ((dSampleDate - dSampleDateGMT) / (1000 * 60 * 60)) // get the GMT for this new hour
if(nCurrentGMToffset != nStandardGMToffset && !bDSTstartDateFound){ // if it is not the same as as the dJanuaryFirstGMT offset, then we know that we are adjusting for DST
document.getElementById('spnDSTstartDate').innerHTML = dSampleDateGMT
nStandardGMToffset = nCurrentGMToffset; //switch the nStandardGMToffset to the DST GMT offset
bDSTstartDateFound = true; //set the boolean flag to true
continue;
}else if(nCurrentGMToffset != nStandardGMToffset){ //this portion will prove true when the GMT offset goes back.(when DST is over)
document.getElementById('spnDSTendDate').innerHTML = dSampleDateGMT
return;
}
}
//If the function makes it this far, there was no adjustment to GMT offset.
document.getElementById('spnDSTstartDate').innerHTML = 'Your locale does not observe Daylight Savings Time';
document.getElementById('spnDSTendDate').innerHTML = 'Your locale does not observe Daylight Savings Time';
}
</script>
</head>
<body>
In your locale, Daylight Savings Time Starts At = <span id="spnDSTstartDate"></span><br>
In your locale, Daylight Savings Time Ends At = <span id="spnDSTendDate"></span>
<script>
displayDSTdates();
</script>
</body>
</html>This code may not work in the southern hemisphere at this time since their seasons are reversed, but I will worry about that after I get this off-by-one bug resolved.
Thanks,
Mike