How to match dateuilts.rrule and python icalendar daylightsavingtime aware

[Gerhard Schmidt]

I'm dealing with some ical problems.

I have an ical event.

BEGIN:VEVENT
UID:Event/termine/gps/[e-mail] 
DTSTART;TZID=CET:20150529T190000
DTEND;TZID=CET:20150529T220000
CATEGORIES:Arbeitsgruppe
DTSTAMP:20110620T075538Z
EXDATE;TZID=CET:20151225T190000
LAST-MODIFIED:20150424T201707Z
LOCATION:Vereinsräume des Augsburger Computer Forum e.V.
PRIORITY:5
RRULE:FREQ=MONTHLY;BYDAY=-1FR
SUMMARY:GPS-Arbeitsgruppe
URL:https://www.augusta.de/termine/gps
END:VEVENT

As you can see there is an RRule repeating this event every last friday of the month.

I parsed this ical with icalendar.

I'm using:

start = iobj.get( 'DTSTART' ).dt
rrset = rruleset()
rrule = iobj.get( 'RRULE' )
exdate = iobj.get( 'EXDATE' )
rrset.rrule( rrule.rrulestr( rule.to_ical(), dtstart = start ) )
for edate in exdate.dts : 
    rrset.exdate( edate.dt )

Everything so far works just fine.

When I Try to get the next say 10 dates with:

list(rrset)[:10] 

I get:

[datetime.datetime(2015, 5, 29, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 6, 26, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 7, 31, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 8, 28, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 9, 25, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 10, 30, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 11, 27, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2015, 12, 25, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2016, 1, 29, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>),
 datetime.datetime(2016, 2, 26, 19, 0, tzinfo=<DstTzInfo 'CET' CEST+2:00:00 DST>)]

Which seems okay on the first glance, but on deeper inspection there is a problem starting with October 30th, daylight saving time ends October 25th but tzinfo info of the datetime object is still "DstTzInfo 'CET' CEST+2:00:00 DST"

The second problem is that the 25th of December is in this list instead of being skipped a specified in the EXDATE. The Problem seams to be tat while parsing the exdate rule daylight saving time is calculated correctly and therefore the exdate 19:00:00+01:00 didn't match the calculated repeat time of 19:00:00+02:00.

Am I doing something wrong there?

Converting everything to UTC and processing there doesn't help because 17:00:00 UTC don't match 18:00:00 UTC either.

[Christian Geier]

This is not really an icalendar problem, more an issue with dateutil's rrule.

Have a look at this stackoverflow issue for a workaround.

[Gerhard Schmidt]

Am 30.05.2015 11:18, schrieb Christian Geier:

This is not really an icalendar problem, more an issue with dateutil's
rrule.

Have a look at this stackoverflow issue
http://stackoverflow.com/questions/25715276/python-daylight-saving-time-issues/25725161#25725161
for a workaround.

This might work as long as all dates are in the same timezone. Having to
deal with date in one timezone but exdate in another will brake things
again.

I will send the problem to dateutil author as there seams to be no
isuetracker or mailingliste for.

Regards
Gerhard

[Christian Geier]

dateutil development has picked up again, here at github:
https://github.com/dateutil/dateutil

Quoting Estartu (2015-05-30 12:10:19)

Am 30.05.2015 11:18, schrieb Christian Geier:

This is not really an icalendar problem, more an issue with dateutil's
rrule.

Have a look at this stackoverflow issue
http://stackoverflow.com/questions/25715276/python-daylight-saving-time-issues/25725161#25725161
for a workaround.

This might work as long as all dates are in the same timezone. Having to
deal with date in one timezone but exdate in another will brake things
again.

I will send the problem to dateutil author as there seams to be no
isuetracker or mailingliste for.

Regards
Gerhard

---

Reply to this email directly or view it on GitHub:
#162 (comment)

[Jonas Einarsson]

I'll reference dateutil/dateutil#641 here.

It seems pytz eager offset calculation causes this error. Icalendar could consider moving off pytz onto dateutil.tz or something else that handles Daylight savings better.

[Christian Geier]

Looks sensible, we should put it onto the next major release schedule.