The deceptively complex world of RRULEs

This is actually what prompted me to start work on luatz: <a href="https:&#x2F;&#x2F;github.com&#x2F;daurnimator&#x2F;luatz" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;daurnimator&#x2F;luatz</a><p>Trying to implement RRULEs you need all sorts of fun date math + converting back and forth between time zones.<p>I never released my ical library (didn&#x27;t get it open sourced from work); but luatz lets you write your own :)

i got tired of all calendars being based on this system, which i find too restricted. You cannot encode many types of holiday, Easter or many religious festivals. You can&#x27;t track the moon either.<p>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 &quot;generator&quot; statement (LUA actually). The script uses a rich set of built in &quot;function kit&quot; such as &quot;fullmoon&quot;, &quot;newmoon&quot; etc. Things like Easter need the moon, and so do many religious festivals.<p>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.<p># Example Generators<p>## Annual date (eg birthdays) return nextMonthDay(d, september, 23)<p>## monthly date, `v&#x27; of month return nextMonth(d)+v-1<p>## first monday of next month return nthkDay(nextMonth(d),1,monday)<p>## 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)<p>next monday on or after next May 1st eg may day return nthkDay(nextMonthDay(d,may,1),1,monday)<p>eg last monday in may, spring bank holiday return nthkDay(nextMonthDay(d+7,june,1),-1,monday)<p>## third tuesday of the month return nthkDay(nextMonth(d),3,tuesday)<p>## fourth thursday in November (Thanksgiving USA) return nthkDay(nextMonthDay(d, november, 1), 4, thursday)<p>## second monday of october (Thanksgiving Canada) return nthkDay(nextMonthDay(d, october, 1), 2, monday)<p>## mothers day UK 4th sunday of Lent = 3 weeks before easter sunday return cal.nextEasterSunday(d+22)-21<p>## 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 &gt; da then return d1 end return d2 end return cvrs(d)<p>## 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))<p>## 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<p>## easter return cal.nextEasterSunday(d)<p>eg good friday return cal.nextEasterSunday(d+3)-2<p>eg. Easter monday return cal.nextEasterSunday(d)+1<p>eg shrove tuesday return cal.nextEasterSunday(d+48)-47<p>##* moon phases return cal.nextNewMoon(d+1) return cal.nextFullMoon(d+1)<p>## ramadan<p>this generates the logical 1&#x2F;9 islamic. Howeever the fast of Ramadan is often started the day earlier if based on the observational moon note that it <i>always</i> begins the sunset beforehand.<p>y,m = cal.islamicFromFixed(d) if m &gt;= 9 then y=y+1 end return cal.fixedFromIslamic(y,9,1)<p>-- end of ramadan is the end of month 9 -- Eid-al-Fitr (Ramadan ends) y,m = cal.islamicFromFixed(d+1) if m &gt;= 10 then y=y+1 end return cal.fixedFromIslamic(y,9,30)<p>-- Al-Hijra (Islamic New Year) return cal.fixedFromIslamic(cal.islamicFromFixed(d)+1,1,1)<p>## Hebrew<p>-- Rosh Hashanah (Jewish New Year) -- 1st of Tishri return cal.fixedFromHebrew(cal.hebrewFromFixed(d)+1,7,1)<p>-- Yom Yippur -- tishri 10 return cal.fixedFromHebrew(cal.hebrewFromFixed(d)+1,7,10)