Re: Working with Dates/Timezones

[Joseph R Lust]

Jens,

Here is the TZ script from my hobby running site. It is a tad hackish (from
4 years ago), set to update a hidden select menu in forms. I have not yet
had any complaints about it's output, but I don't promise anything either.
Hopefully you might find it helpful and can make a JSNI derivative if it
meets your needs.


Sincerely,
Joseph

// original script by Josh Fraser (http://www.onlineaspect.com)
function calculate_time_zone() {
    var rightNow = new Date();
    var jan1 = new Date(rightNow.getFullYear(), 0, 1, 0, 0, 0, 0);  // jan 1st
    var june1 = new Date(rightNow.getFullYear(), 6, 1, 0, 0, 0, 0); // june 1st
    var temp = jan1.toGMTString();
    var jan2 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
    temp = june1.toGMTString();
    var june2 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
    var std_time_offset = (jan1 - jan2) / (1000 * 60 * 60);
    var daylight_time_offset = (june1 - june2) / (1000 * 60 * 60);
    var dst;
    if (std_time_offset == daylight_time_offset) {
        dst = "0"; // daylight savings time is NOT observed
    } else {
        // positive is southern, negative is northern hemisphere
        var hemisphere = std_time_offset - daylight_time_offset;
        if (hemisphere >= 0)
            std_time_offset = daylight_time_offset;
        dst = "1"; // daylight savings time is observed
    }
    var i;
    // check just to avoid error messages (hidden select posted with forms)
    if (document.getElementById('timezone_name')) {
        for (i = 0; i <
document.getElementById('timezone_name').options.length; i++) {
            if (document.getElementById('timezone_name').options[i].value
== convert(std_time_offset)+","+dst) {
                document.getElementById('timezone_name').selectedIndex = i;
                break;
            }
        }
    }
}

function convert(value) {
    var hours = parseInt(value);
       value -= parseInt(value);
    value *= 60;
    var mins = parseInt(value);
       value -= parseInt(value);
    value *= 60;
    var display_hours = hours;
    // handle GMT case (00:00)
    if (hours == 0) {
        display_hours = "00";
    } else if (hours > 0) {
        // add a plus sign and perhaps an extra 0
        display_hours = (hours < 10) ? "+0"+hours : "+"+hours;
    } else {
        // add an extra 0 if needed
        display_hours = (hours > -10) ? "-0"+Math.abs(hours) : hours;
    }

    mins = (mins < 10) ? "0"+mins : mins;
    return display_hours+":"+mins;
}

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