<!DOCTYPE html>
<html>
<head>
<title>DST Calculator</title>
<script type="text/javascript">
function DisplayDstSwitchDates()
{
var year = new Date().getYear();
if (year < 1000)
year += 1900;
var firstSwitch = 0;
var secondSwitch = 0;
var lastOffset = 99;
// Loop through every month of the current year
for (i = 0; i < 12; i++)
{
// Fetch the timezone value for the month
var newDate = new Date(Date.UTC(year, i, 0, 0, 0, 0, 0));
var tz = -1 * newDate.getTimezoneOffset() / 60;
// Capture when a timzezone change occurs
if (tz > lastOffset)
firstSwitch = i-1;
else if (tz < lastOffset)
secondSwitch = i-1;
lastOffset = tz;
}
// Go figure out date/time occurences a minute before
// a DST adjustment occurs
var secondDstDate = FindDstSwitchDate(year, secondSwitch);
var firstDstDate = FindDstSwitchDate(year, firstSwitch);
if (firstDstDate == null && secondDstDate == null)
return 'Daylight Savings is not observed in your timezone.';
else
return 'Last minute before DST change occurs in ' +
year + ': ' + firstDstDate + ' and ' + secondDstDate;
}
function FindDstSwitchDate(year, month)
{
// Set the starting date
var baseDate = new Date(Date.UTC(year, month, 0, 0, 0, 0, 0));
var changeDay = 0;
var changeMinute = -1;
var baseOffset = -1 * baseDate.getTimezoneOffset() / 60;
var dstDate;
// Loop to find the exact day a timezone adjust occurs
for (day = 0; day < 50; day++)
{
var tmpDate = new Date(Date.UTC(year, month, day, 0, 0, 0, 0));
var tmpOffset = -1 * tmpDate.getTimezoneOffset() / 60;
// Check if the timezone changed from one day to the next
if (tmpOffset != baseOffset)
{
var minutes = 0;
changeDay = day;
// Back-up one day and grap the offset
tmpDate = new Date(Date.UTC(year, month, day-1, 0, 0, 0, 0));
tmpOffset = -1 * tmpDate.getTimezoneOffset() / 60;
// Count the minutes until a timezone chnage occurs
while (changeMinute == -1)
{
tmpDate = new Date(Date.UTC(year, month, day-1, 0, minutes, 0, 0));
tmpOffset = -1 * tmpDate.getTimezoneOffset() / 60;
// Determine the exact minute a timezone change
// occurs
if (tmpOffset != baseOffset)
{
// Back-up a minute to get the date/time just
// before a timezone change occurs
tmpOffset = new Date(Date.UTC(year, month,
day-1, 0, minutes-1, 0, 0));
changeMinute = minutes;
break;
}
else
minutes++;
}
// Add a month (for display) since JavaScript counts
// months from 0 to 11
dstDate = tmpOffset.getMonth() + 1;
// Pad the month as needed
if (dstDate < 10) dstDate = "0" + dstDate;
// Add the day and year
dstDate += '/' + tmpOffset.getDate() + '/' + year + ' ';
// Capture the time stamp
tmpDate = new Date(Date.UTC(year, month,
day-1, 0, minutes-1, 0, 0));
dstDate += tmpDate.toTimeString().split(' ')[0];
return dstDate;
}
}
}
</script>
</head>
<body>
<script type="text/javascript">
document.write("Current date/time: " + new Date() + "<br />");
document.write(DisplayDstSwitchDates());
</script>
</body>
</html>
