The deceptively complex world of RRULEs

Trying to implement RRULEs you need all sorts of fun date math + converting back and forth between time zones.

I never released my ical library (didn't get it open sourced from work); but luatz lets you write your own :)

So, i wrote my own calendar and the events work as a start datetime, end datetime and optional recurrence. the recurrence is encoded as a small script "generator" statement (LUA actually). The script uses a rich set of built in "function kit" such as "fullmoon", "newmoon" etc. Things like Easter need the moon, and so do many religious festivals.

The generator returns the NEXT datetime, so that the calendar can chain these events so that scrolling works nicely and you only calculate what you need.

# Example Generators

## Annual date (eg birthdays) return nextMonthDay(d, september, 23)

## monthly date, `v' of month return nextMonth(d)+v-1

## first monday of next month return nthkDay(nextMonth(d),1,monday)

## last sunday in a month next year eg last sunday next march. finds next april 1 and works back to last sunday return nthkDay(nextMonthDay(d+7,april,1),-1,sunday)

next monday on or after next May 1st eg may day return nthkDay(nextMonthDay(d,may,1),1,monday)

eg last monday in may, spring bank holiday return nthkDay(nextMonthDay(d+7,june,1),-1,monday)

## third tuesday of the month return nthkDay(nextMonth(d),3,tuesday)

## fourth thursday in November (Thanksgiving USA) return nthkDay(nextMonthDay(d, november, 1), 4, thursday)

## second monday of october (Thanksgiving Canada) return nthkDay(nextMonthDay(d, october, 1), 2, monday)

## mothers day UK 4th sunday of Lent = 3 weeks before easter sunday return cal.nextEasterSunday(d+22)-21

## first and third thursday of the month function cvrs(da) local d1 = nthkDay(thisMonth(da),3,thursday) local d2 = nthkDay(nextMonth(da),1,thursday) if d1 > da then return d1 end return d2 end return cvrs(d)

## first weekday of month function weekday(d) local da = cal.dayOfWeek(d) if da == sunday then d = d + 1 elseif da == saturday then d = d + 2 end return d end return weekday(nextMonth(d))

## first weekday on or before given date of of month function weekdayb4(d) local da = cal.dayOfWeek(d) if da == sunday then d = d - 2 elseif da == saturday then d = d - 1 end return d end return weekdayb4(nextMonth(d)+25) -- example

## easter return cal.nextEasterSunday(d)

eg good friday return cal.nextEasterSunday(d+3)-2

eg. Easter monday return cal.nextEasterSunday(d)+1

eg shrove tuesday return cal.nextEasterSunday(d+48)-47

##* moon phases return cal.nextNewMoon(d+1) return cal.nextFullMoon(d+1)

## ramadan

this generates the logical 1/9 islamic. Howeever the fast of Ramadan is often started the day earlier if based on the observational moon note that it always begins the sunset beforehand.

y,m = cal.islamicFromFixed(d) if m >= 9 then y=y+1 end return cal.fixedFromIslamic(y,9,1)

-- end of ramadan is the end of month 9 -- Eid-al-Fitr (Ramadan ends) y,m = cal.islamicFromFixed(d+1) if m >= 10 then y=y+1 end return cal.fixedFromIslamic(y,9,30)

-- Al-Hijra (Islamic New Year) return cal.fixedFromIslamic(cal.islamicFromFixed(d)+1,1,1)

## Hebrew

-- Rosh Hashanah (Jewish New Year) -- 1st of Tishri return cal.fixedFromHebrew(cal.hebrewFromFixed(d)+1,7,1)

-- Yom Yippur -- tishri 10 return cal.fixedFromHebrew(cal.hebrewFromFixed(d)+1,7,10)