Get Moon Phase by Date, written in JavaScript

getMoonPhase.js

/* 
 * modified from http://www.voidware.com/moon_phase.htm 
 */

function getMoonPhase(year, month, day)
{
    var c = e = jd = b = 0;

    if (month < 3) {
        year--;
        month += 12;
    }

    ++month;

    c = 365.25 * year;

    e = 30.6 * month;

    jd = c + e + day - 694039.09; //jd is total days elapsed

    jd /= 29.5305882; //divide by the moon cycle

    b = parseInt(jd); //int(jd) -> b, take integer part of jd

    jd -= b; //subtract integer part to leave fractional part of original jd

    b = Math.round(jd * 8); //scale fraction from 0-8 and round

    if (b >= 8 ) {
        b = 0; //0 and 8 are the same so turn 8 into 0
    }

    // 0 => New Moon
    // 1 => Waxing Crescent Moon
    // 2 => Quarter Moon
    // 3 => Waxing Gibbous Moon
    // 4 => Full Moon
    // 5 => Waning Gibbous Moon
    // 6 => Last Quarter Moon
    // 7 => Waning Crescent Moon
    
    return b;
}

console.log(getMoonPhase(2015, 3, 29));

Full moon was last night here, there are regional differences, but not by that much. You can check the exact time of the full moon for your area with an ephemeris such as this one: https://in-the-sky.org/ephemeris.php?ird=1&irs=1&ima=1&iph=1&iob=1&objtype=1&objpl=Moon&objtxt=the+Moon&tz=0&startday=1&startmonth=6&startyear=2022&interval=1&rows=100

And then cross-reference with the scripts results.