Import .ics files exported from Zimbra Collaboration Suite into Google Calendar

I ran into an issue today where the iCalendar file I exported from Zimbra Collaboration Suite (ZCS) would not import into Google Calendar. It would import into Apple's iCal, and into Mozilla's Sunbird just fine, but Google Calendar just sat there. Also, when I imported the calendar into a Google caldav account I'd set up in iCal, iCal kept freezing.

My solution was to import the .ics file I'd exported from ZCS into a local calendar in iCal, then export the calendar from iCal and import it into Google Calendar. The VCalendar headers for both files are different:

The header of the .ics file directly from zimbra:

BEGIN:VCALENDAR
PRODID:Zimbra-Calendar-Provider
VERSION:2.0
METHOD:PUBLISH
BEGIN:VTIMEZONE
TZID:America/Los_Angeles
BEGIN:STANDARD

The header of the .ics file after exporting from iCal: (I did change the name of "X-WR-CALNAME" as it was a user's name instead of "Imported)

BEGIN:VCALENDAR
METHOD:PUBLISH
X-WR-TIMEZONE:US/Pacific
PRODID:-//Apple Inc.//iCal 3.0//EN
CALSCALE:GREGORIAN
X-WR-CALNAME:Imported
VERSION:2.0
X-WR-RELCALID:5A28F3E9-A17C-46E1-9DB3-3DEC935C5595
X-APPLE-CALENDAR-COLOR:#E51717
BEGIN:VTIMEZONE
TZID:US/Central
BEGIN:DAYLIGHT
TZOFFSETFROM:-0600
TZOFFSETTO:-0500
DTSTART:20070311T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
TZNAME:CDT
END:DAYLIGHT
BEGIN:STANDARD

I will have to go research it a bit more to know which part is standard, and which is included by Zimbra or iCal, but if Google Calendar is looking for something in the ics header that wasn't present in the zimbra-generated one, then there's the problem, I suppose. One main interesting thing, is the timezone difference. I've not set iCal to be set for the central timezone. In fact I had erased all of my iCal preferences for that user account before I did this. I just went back and checked iCal's preferences, and timezone support wasn't enabled; the calendar did seem like it was conforming to my local clock's settings.