Hebrew Calender API/Library in PHP or Javascript to add days, month or years to Julian date

Question

I have a web project where I need a function either server-side using php or client-side using javascript where I can pass in a Georgian date and have month(s) or years added to it according to the Hebrew calendar, and return a Julian date.

for example the date 12/8/2014 which is 15 Kislev when adding a month it should return 1/7/2014 which is the Hebrew date of 15 Tevet instead of 1/8/2014 which is returned by regular date add functions which is actually 16 Tevet.

I've searched around and couldn't find anything, Hebcal doesn't seem to have any of these params.

Answer 1

Check out my hebcal-js library, the perfect thing you need for this. Include it, and then you can use one of the following snippets:

This is only if you will never have dates in Elul (due to a bug):

var hebDate = new Hebcal.HDate(new Date(2014, 11 /* meaning 12 */, 8));
hebDate.setMonth(hebDate.getMonth() + 1);
var gregDate = hebDate.greg(); // a javascript Date object

This will work for any date:

var hebDate = new Hebcal.HDate(new Date(2014, 11 /* meaning 12 */, 8));
var gregDate = hebDate.getMonthObject().next().getDay(hebDate.getDate()).greg(); // a javascript Date object

Answer 2

Looking at your example, you can simply take advantage of the date conversion functions built into whichever library you are using. So if you want to add a Hebrew month to a Julian date, you can:

1. Convert the Julian date into a Hebrew date (i.e. Hebrew Year / Month / Day)
2. Take the Hebrew month value and add 1. Just keep in mind that if you are in Elul and add 1, you end up in Tishrei of the next year (the year wraps). Also, only allow it to go to Adar Sheini if it is a Hebrew leap year (once again, your library should be able to tell you if it is)
3. Now use a function to convert this new Hebrew date (made up of y/m/d) back into Julian

Here's some pseudo-ish code (not tested):

function addAHebrewMonth(julianDate):
    hebrew_year, hebrew_month, hebrew_day = julianToHebrew(julianDate)
    hebrew_month += 1
    if hebrew_month == 7: //Tishrei wrap
        hebrew_year += 1

    //Adar II (this will depend on how your library deals with leap years...)
    if hebrew_month == 13 and not isHebrewLeapYear(hebrew_year):
        hebrew_month = 1 //wrap back to Nisan

    return hebrewToJulian(hebrew_year, hebrew_month, hebrew_day)

Voila!