Calculate UTC offset at a given time from VTIMEZONE Block provided in a .ics file

Question

My application parses supplied .ics files and fetches the start and end time of a calendar event in UTC. See example .ics file below.

BEGIN:VCALENDAR
BEGIN:VTIMEZONE
TZID:
BEGIN:STANDARD
DTSTART:16010101T020000
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=1SU;BYMONTH=11
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010101T020000
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=2SU;BYMONTH=3
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ORGANIZER;CN=blah:MAILTO:someone@blah.com
DESCRIPTION;LANGUAGE=en-US:
RRULE:FREQ=YEARLY;COUNT=30;INTERVAL=1;BYMONTHDAY=9;BYMONTH=4
SUMMARY;LANGUAGE=en-US: This is summary
DTSTART;TZID=:20160409T000000
DTEND;TZID=:20160410T000000
UID:f663243d-17f2-424a-a52c
RECURRENCE-ID;TZID=:20160409T000000
PRIORITY:5
DTSTAMP:20150701T123848Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION;LANGUAGE=en-US:
END:VEVENT
END:VCALENDAR

I have come across situations where there is no TZID mentioned with DTSTART/DTEND (as in the above .ics file), or the TZID is some random string from which you cannot figure out the timezone.

I think the only way to get timezone info in such situations is to parse the VTIMEZONE block to determine the timezone offset and then apply it to the start and end time to get the value in UTC.

Can someone please help me with the algorithm (or point to an existing library) to parse the VTIMEZONE block and get the UTC offset at given time.

EDIT: I am working on python. So a python library would be great. It's OK if you suggest in some other language too, all I need is the algorithm. I can convert it to working code myself.

Answer 1

I don't know what language you're using, but biweekly is a Java library that parses iCalendar files. It can also parse the timezone information out of VTIMEZONE components.