Calculate sunrise/sunset times from latitude/longitude coordinates using Javascript.

suntimes.js

/**
* Calculates today's sunrise and sunset hours in local time (or in the given tz) for the given latitude, longitude.
*   The tz parameter is mainly for the possible circumstance that your system timezone does not match the location
*   you are currently at.
*
* Computations are based on the formulas found in:
*   https://en.wikipedia.org/wiki/Julian_day#Converting_Julian_or_Gregorian_calendar_date_to_Julian_Day_Number
*   https://en.wikipedia.org/wiki/Sunrise_equation#Complete_calculation_on_Earth
*
* @method suntimes
* @param {Float} lat Latitude of location (South is negative)
* @param {Float} lng Longitude of location (West is negative)
* @param {Integer} tz Timezone hour offset. e.g. Pacific/Los Angeles Standard Time is -8 (Optional, defaults to system timezone)
* @return {Array} Returns an array of length 2 with the sunrise and sunset times as floats on 24-hour time.
*                    e.g. 6.5 is 6:30am, 23.2 is 11:12pm, 0.3 is 12:18am
*                 Returns an array with [null, -1] if the sun never rises, and [-1, null] if the sun never sets.
*
* ### LICENSING ###
* 
* This code contains information derived from Wikipedia content licensed under
*   the Creative Commons Attribution-ShareAlike License. As such, this code
*   is distributed under the GPLv3 license:
*
* This program is free software: you can redistribute it and/or modify
*   it under the terms of the GNU General Public License as published by
*   the Free Software Foundation, either version 3 of the License, or
*   (at your option) any later version.
*
*   This program is distributed in the hope that it will be useful,
*   but WITHOUT ANY WARRANTY; without even the implied warranty of
*   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
*   GNU General Public License for more details.
*
*   You should have received a copy of the GNU General Public License
*   along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

function suntimes(lat, lng, tz) {
    var d = new Date();
    var radians = Math.PI / 180.0;
    var degrees = 180.0 / Math.PI;

    var a = Math.floor((14 - (d.getMonth() + 1.0)) / 12)
    var y = d.getFullYear() + 4800 - a;
    var m = (d.getMonth() + 1) + 12 * a - 3;
    var j_day = d.getDate() + Math.floor((153 * m + 2)/5) + 365 * y + Math.floor(y/4) - Math.floor(y/100) + Math.floor(y/400) - 32045;
    var n_star = j_day - 2451545.0009 - lng / 360.0;
    var n = Math.floor(n_star + 0.5);
    var solar_noon = 2451545.0009 - lng / 360.0 + n;
    var M = 356.0470 + 0.9856002585 * n;
    var C = 1.9148 * Math.sin( M * radians ) + 0.02 * Math.sin( 2 * M * radians ) + 0.0003 * Math.sin( 3 * M * radians );
    var L = ( M + 102.9372 + C + 180 ) % 360;
    var j_transit = solar_noon + 0.0053 * Math.sin( M * radians) - 0.0069 * Math.sin( 2 * L * radians );
    var D = Math.asin( Math.sin( L * radians ) * Math.sin( 23.45 * radians ) ) * degrees;
    var cos_omega = ( Math.sin(-0.83 * radians) - Math.sin( lat * radians ) * Math.sin( D * radians ) ) / ( Math.cos( lat * radians ) * Math.cos( D * radians ) );

    // sun never rises
    if( cos_omega > 1)
      return [null, -1];

    // sun never sets
    if( cos_omega < -1 )
      return [-1, null];

    // get Julian dates of sunrise/sunset
    var omega = Math.acos( cos_omega ) * degrees;
    var j_set = j_transit + omega / 360.0;
    var j_rise = j_transit - omega / 360.0;

    /*
    * get sunrise and sunset times in UTC
    * Check section "Finding Julian date given Julian day number and time of
    *  day" on wikipedia for where the extra "+ 12" comes from.
    */
    var utc_time_set = 24 * (j_set - j_day) + 12;
    var utc_time_rise = 24 * (j_rise - j_day) + 12;
    var tz_offset = tz === undefined ? -1 * d.getTimezoneOffset() / 60 : tz;
    var local_rise = (utc_time_rise + tz_offset) % 24;
    var local_set = (utc_time_set + tz_offset) % 24;
    return [local_rise, local_set];
}

I saw your docs on tz:

* @param {Integer} tz Timezone hour offset. e.g. Pacific/Los Angeles is -8 (Optional, defaults to system timezone)

And I'm in LA and when I give it -8 as a parameter it gives a different result. Seems to be because (new Date()).getTimezoneOffset()/60 is 7 and not -8. I'm guessing the 1hr difference is daylight savings and the negative vs positive is because we're UTC-8 but we're 8 (now 7) hours behind)

Thanks for mentioning this. I have specified that -8 is PST in the docs.