# Module:Next Occurrence of Hebrew Date

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## Usage

```{{#invoke:Next Occurrence of Hebrew Date|next_occurrence_of_hebrew_date_from_date | greg_year | greg_month | greg_day | heb_month | heb_day}}
```

Where:

• `greg_year` is the Gregorian year, like 2014
• `greg_month` is the Gregorian month number, like 12 for December
• `greg_day` is the Gregorian day of month
• `heb_month` can either be the Hebrew month, like Adar, or it's number, like 12 for Adar
• `heb_day` is the Hebrew day of month

So for example, if you wanted to compute when the day of Rosh Hashanah falls out on in 2015:

```{{#invoke:Next Occurrence of Hebrew Date|next_occurrence_of_hebrew_date_from_date | 2015 | 1 | 1 | 7 | 1}}
```

which returns September 14, 2015

There is also a convenience function, next_occurrence_of_hebrew_date, which performs the same calculation, with a start time of today's date. This is useful for determining the next occurrence of a Jewish holiday or Yahrtzeit.

```{{#invoke:Next Occurrence of Hebrew Date|next_occurrence_of_hebrew_date_from_date |heb_month| heb_day}}
```

Where:

• `heb_month` can either be the Hebrew month, like Adar, or it's number, like 12 for Adar
• `heb_day` is the Hebrew day of month

So for example, if you wanted to compute when the next Tu B'shvat falls out on, from the current date:

```{{#invoke:Next Occurrence of Hebrew Date|next_occurrence_of_hebrew_date|Shevat|15}}
```

which returns February 13, 2025

```-- Based on public domain code from: http://emr.cs.iit.edu/~reingold/calendar.C

local p = {}
-- Absolute dates

-- "Absolute date" means the number of days elapsed since the Gregorian date
-- Sunday, December 31, 1 BC. (Since there was no year 0, the year following
-- 1 BC is 1 AD.) Thus the Gregorian date January 1, 1 AD is absolute date
-- number 1.

--  Gregorian dates

function LastDayOfGregorianMonth(month, year)
-- Compute the last date of the month for the Gregorian calendar.

if month == 2 then
if ((((year % 4) == 0) and ((year % 100) ~= 0)) or ((year % 400) == 0)) then
return 29;
else
return 28;
end
elseif month == 4 or month == 6 or month == 9 or month == 11 then
return 30;
else
return 31;
end
end

GregorianDate = {}
GregorianDate.__index = GregorianDate

function GregorianDate.create(month, day, year)
local gregoriandate = {}             -- our new object
setmetatable(gregoriandate,GregorianDate)  -- make GregorianDate handle lookup
-- initialize our object
gregoriandate.month = month --  1 == January, ..., 12 == December
gregoriandate.day = day     -- 1..LastDayOfGregorianMonth(month, year)
gregoriandate.year = year   --
return gregoriandate
end

function GregorianDateFromAbsolute(d)
-- Computes the Gregorian date from the absolute date.

-- Search forward year by year from approximate year
local year = math.floor(d/366);
while (d >= AbsoluteFromGregorianDate(GregorianDate.create(1,1,year+1))) do
year = year + 1;
end

-- Search forward month by month from January
local month = 1;
while (d > AbsoluteFromGregorianDate(GregorianDate.create(month, LastDayOfGregorianMonth(month,year), year))) do
month = month + 1;
end

local day = d - AbsoluteFromGregorianDate(GregorianDate.create(month,1,year)) + 1;

return GregorianDate.create(month, day, year)
end

function AbsoluteFromGregorianDate(gregoriandate)
-- Computes the absolute date from the Gregorian date.
local N = gregoriandate.day;           -- days this month

-- days in prior months this year
local m = gregoriandate.month - 1;
while (m > 0) do
N = N + LastDayOfGregorianMonth(m, gregoriandate.year);
m = m - 1;
end

return (N                    -- days this year
+ 365 * (gregoriandate.year - 1)   -- days in previous years ignoring leap days
+ math.floor((gregoriandate.year - 1)/4)       -- Julian leap days before this year...
- math.floor((gregoriandate.year - 1)/100)     -- ...minus prior century years...
+ math.floor((gregoriandate.year - 1)/400));   -- ...plus prior years divisible by 400

end

-- Hebrew dates

HebrewEpoch = -1373429 -- Absolute date of start of Hebrew calendar

function HebrewLeapYear(year)
-- True if year is an Hebrew leap year
if ((((7 * year) + 1) % 19) < 7)
then
return 1;
else
return 0;
end
end

--Last month of Hebrew year.
function LastMonthOfHebrewYear(year)
if (HebrewLeapYear(year) == 1) then
return 13;
else
return 12;
end
end

function HebrewCalendarElapsedDays(year)
-- Number of days elapsed from the Sunday prior to the start of the
-- Hebrew calendar to the mean conjunction of Tishri of Hebrew year.

local MonthsElapsed =
(235 * math.floor((year - 1) / 19))           -- Months in complete cycles so far.
+ (12 * ((year - 1) % 19))          -- Regular months in this cycle.
+ math.floor((7 * ((year - 1) % 19) + 1) / 19); -- Leap months this cycle
local PartsElapsed = 204 + 793 * (MonthsElapsed % 1080);
local HoursElapsed =
5 + 12 * MonthsElapsed + 793 * math.floor(MonthsElapsed  / 1080)
+ math.floor(PartsElapsed / 1080);
local ConjunctionDay = 1 + 29 * MonthsElapsed + math.floor(HoursElapsed / 24);
local ConjunctionParts = 1080 * (HoursElapsed % 24) + PartsElapsed % 1080;
local AlternativeDay = 0;
if ((ConjunctionParts >= 19440)        -- If new moon is at or after midday,
or (((ConjunctionDay % 7) == 2)    -- ...or is on a Tuesday...
and (ConjunctionParts >= 9924)  -- at 9 hours, 204 parts or later...
and (HebrewLeapYear(year)) == 0)   -- ...of a common year,
or (((ConjunctionDay % 7) == 1)    -- ...or is on a Monday at...
and (ConjunctionParts >= 16789) -- 15 hours, 589 parts or later...
and (HebrewLeapYear(year - 1) == 1))) then -- at the end of a leap year
-- Then postpone Rosh HaShanah one day
AlternativeDay = ConjunctionDay + 1;
else
AlternativeDay = ConjunctionDay;
end

if (((AlternativeDay % 7) == 0)-- If Rosh HaShanah would occur on Sunday,
or ((AlternativeDay % 7) == 3)     -- or Wednesday,
or ((AlternativeDay % 7) == 5))    -- or Friday
-- Then postpone it one (more) day
then
return (1+ AlternativeDay);
else
return AlternativeDay;
end
end

function DaysInHebrewYear(year)
-- Number of days in Hebrew year.

return ((HebrewCalendarElapsedDays(year + 1)) -
(HebrewCalendarElapsedDays(year)));
end

function LongHeshvan(year)
-- True if Heshvan is long in Hebrew year.

if ((DaysInHebrewYear(year) % 10) == 5)
then
return 1;
else
return 0;
end
end

function ShortKislev(year)
-- True if Kislev is short in Hebrew year.

if ((DaysInHebrewYear(year) % 10) == 3)
then
return 1;
else
return 0;
end
end

function LastDayOfHebrewMonth(month, year)
-- Last day of month in Hebrew year.

if ((month == 2)
or (month == 4)
or (month == 6)
or ((month == 8) and LongHeshvan(year) == 0)
or ((month == 9) and ShortKislev(year) == 1)
or (month == 10)
or ((month == 12) and (HebrewLeapYear(year) == 0))
or (month == 13))
then
return 29;
else
return 30;
end
end

HebrewDate = {}
HebrewDate.__index = HebrewDate

function HebrewDate.create(month, day, year)
local hebrewdate = {}             -- our new object
setmetatable(hebrewdate,HebrewDate)  -- make HebrewDate handle lookup
-- initialize our object
hebrewdate.month = month -- 1...
hebrewdate.day = day     -- 1..LastMonthOfHebrewYear(year)
hebrewdate.year = year   -- 1..LastDayOfHebrewMonth(month, year)
return hebrewdate
end

function HebrewDateFromAbsolute(d)
-- Computes the Hebrew date from the absolute date.
local year = math.floor((d + HebrewEpoch) / 366); -- Approximation from below.
-- Search forward for year from the approximation.
while (d >= AbsoluteFromHebrewDate(HebrewDate.create(7,1,year + 1)))
do
year = year + 1;
end
-- Search forward for month from either Tishri or Nisan.
local month = 0
if (d < AbsoluteFromHebrewDate(HebrewDate.create(1, 1, year))) then
month = 7;  --  Start at Tishri
else
month = 1;  --  Start at Nisan
end

while (d > AbsoluteFromHebrewDate(HebrewDate.create(month, (LastDayOfHebrewMonth(month,year)), year))) do
month = month + 1;
end
-- Calculate the day by subtraction.
local day = d - AbsoluteFromHebrewDate(HebrewDate.create(month, 1, year)) + 1;
return HebrewDate.create(month, day, year)
end

function AbsoluteFromHebrewDate(hebrewdate)
-- Computes the absolute date of Hebrew date.
local DayInYear = hebrewdate.day; -- Days so far this month.
if (hebrewdate.month < 7) then -- Before Tishri, so add days in prior months
-- this year before and after Nisan.
local m = 7;
while (m <= (LastMonthOfHebrewYear(hebrewdate.year))) do
DayInYear = DayInYear + LastDayOfHebrewMonth(m, hebrewdate.year);
m = m + 1;
end

m = 1;
while (m < hebrewdate.month) do
DayInYear = DayInYear + LastDayOfHebrewMonth(m, hebrewdate.year);
m = m + 1;
end

else  -- Add days in prior months this year
local m = 7;
while (m < hebrewdate.month) do
DayInYear = DayInYear + LastDayOfHebrewMonth(m, hebrewdate.year);
m = m + 1;
end
end
return (DayInYear +
(HebrewCalendarElapsedDays(hebrewdate.year)-- Days in prior years.
+ HebrewEpoch));         -- Days elapsed before absolute date 1.
end

function find_gregorian_for_next_hebrew_date_occurrence(greg_year, greg_month, greg_day, heb_month, heb_day)
local greg_absolute = AbsoluteFromGregorianDate(GregorianDate.create(greg_month, greg_day, greg_year))
local hebrew_date = HebrewDateFromAbsolute(greg_absolute)

local heb_year = hebrew_date.year

-- Check if we already passed that date this year. If we have, increase the year by 1
local this_years_hebrew_absolute = AbsoluteFromHebrewDate(HebrewDate.create(heb_month, heb_day, heb_year))
if (greg_absolute > this_years_hebrew_absolute) then
heb_year = heb_year + 1
end

-- Certain months only have 29 days. Advance years until we find a year with 30 days that month.
if heb_day == 30 then
while ((heb_month == 8) and not(LongHeshvan(heb_year))) or ((heb_month == 9) and ShortKislev(heb_year)) or ((heb_month == 12) and (HebrewLeapYear(heb_year) == 0)) do
heb_year = heb_year + 1
end
end

-- The year we're in has the date. Get the absolute and convert it back to a Gregorian date
local absolute = AbsoluteFromHebrewDate(HebrewDate.create(heb_month, heb_day, heb_year))
local gregorian = GregorianDateFromAbsolute(absolute)
return gregorian
end

function GregorianMonthToName(monthNumber)
if monthNumber == 1 then
return "January"
elseif monthNumber == 2 then
return "February"
elseif monthNumber == 3 then
return "March"
elseif monthNumber == 4 then
return "April"
elseif monthNumber == 5 then
return "May"
elseif monthNumber == 6 then
return "June"
elseif monthNumber == 7 then
return "July"
elseif monthNumber == 8 then
return "August"
elseif monthNumber == 9 then
return "September"
elseif monthNumber == 10 then
return "October"
elseif monthNumber == 11 then
return "November"
elseif monthNumber == 12 then
return "December"
end
end

function HebrewMonthNameToNumber(month)
month = string.lower(month)
if month == "nisan" then
return 1
elseif month == "iyar" then
return 2
elseif month == "sivan" then
return 3
elseif month == "tammuz" then
return 4
elseif month == "av" then
return 5
elseif month == "elul" then
return 6
elseif month == "tishrei" then
return 7
elseif month == "cheshvan" then
return 8
elseif month == "kislev" then
return 9
elseif month == "tevet" then
return 10
elseif month == "shevat" then
return 11
return 12
else
return nil
end
end

function p.next_occurrence_of_hebrew_date_from_date(frame)
local greg_year = tonumber(frame.args[1])
local greg_month = tonumber(frame.args[2])
local greg_day = tonumber(frame.args[3])
local heb_month = frame.args[4]
local heb_day = tonumber(frame.args[5])
-- Validate some of the parameters:
-- heb_month can either be a month name or number
heb_month_number = HebrewMonthNameToNumber(heb_month)
if heb_month_number then
heb_month = heb_month_number
else
heb_month = tonumber(heb_month)
end

gregorian = find_gregorian_for_next_hebrew_date_occurrence(greg_year, greg_month, greg_day, heb_month, heb_day)
month_name = GregorianMonthToName(gregorian.month)
return month_name.." ".. gregorian.day..", "..gregorian.year
end

function p.next_occurrence_of_hebrew_date(frame)
local greg_year = tonumber(os.date("%Y"))
local greg_month = tonumber(os.date("%m"))
local greg_day = tonumber(os.date("%d"))
local heb_month = frame.args[1]
local heb_day = tonumber(frame.args[2])
-- Validate some of the parameters:
-- heb_month can either be a month name or number
heb_month_number = HebrewMonthNameToNumber(heb_month)
if heb_month_number then
heb_month = heb_month_number
else
heb_month = tonumber(heb_month)
end

gregorian = find_gregorian_for_next_hebrew_date_occurrence(greg_year, greg_month, greg_day, heb_month, heb_day)
month_name = GregorianMonthToName(gregorian.month)
return month_name.." ".. gregorian.day..", "..gregorian.year
end

return p
```