Test for this example

[jarofgreen]

http://www.kanzaki.com/docs/ical/rrule.html

All examples assume the Eastern United States time zone.

Every 20 minutes from 9:00 AM to 4:40 PM every day:

DTSTART;TZID=US-Eastern:19970902T090000
RRULE:FREQ=DAILY;BYHOUR=9,10,11,12,13,14,15,16;BYMINUTE=0,20,40
or
RRULE:FREQ=MINUTELY;INTERVAL=20;BYHOUR=9,10,11,12,13,14,15,16

==> (September 2, 1997 EDT)9:00,9:20,9:40,10:00,10:20,
... 16:00,16:20,16:40
(September 3, 1997 EDT)9:00,9:20,9:40,10:00,10:20,
...16:00,16:20,16:40
...

[jarofgreen]

Just noticed this example is from an old RFC that has now been preceded by RFC 5545; check it's still valid!