Recurring rules

[d1g]

Some non-core libraries support recurrence in time, it is good idea to support export to them

I wasn't able to find good study of libraries, some random libs below (suggest missing if they are better):

https://tools.ietf.org/html/rfc5545 (updated 3 times)
https://github.com/antlr/grammars-v4/tree/master/icalendar - grammar
http://icalendar.org/resources.html note "Programming Libraries"
https://en.wikipedia.org/wiki/List_of_applications_with_iCalendar_support
https://github.com/ical4j/ical4j/wiki#related-sites-and-articles

C/C++

http://libical.github.io/libical/

Java

https://github.com/ical4j/ical4j/wiki

JavaScript

https://github.com/jkbrzt/rrule
http://momentjs.com/docs/#/plugins/recur/ - only as plugin

PHP

http://kigkonsult.se/iCalcreator/index.php

Python

https://pypi.python.org/pypi/icalendar

Ruby

https://github.com/icalendar/icalendar
https://github.com/mlipper/runt

PL/pgSQL

https://github.com/bakineggs/recurring_events_for