Outlook 2003 does not import .ics calendar while outlook 2007 does

I have an .ics file I manually create with PHP like the following:

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 11.0 MIMEDIR//EN
VERSION:2.0
METHOD:PUBLISH
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
CLASS:PUBLIC
DESCRIPTION:Some description here
LOCATION:Scheduled PTO
DTEND;VALUE=DATE:20101104
DTSTART;VALUE=DATE:20101103
PRIORITY:5
SEQUENCE:0
SUMMARY;LANGUAGE=en-us:PTO - 8.00 hour(s)
X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
X-MICROSOFT-MSNCALENDAR-ALLDAYEVENT:TRUE
END:VEVENT
END:VCALENDAR

This works fine when importing into outlook 2007 (as well as Google calendar). In outlook 2003 I get the following error:

"This error can appear if you have attempted to save a recurring Lunar Calendar in iCalendar format. To avoid this error, set the appointment option to Gregorian instead of Lunar."

Is there something I need to change in the .ics file to get this to work with Outlook 2003?

(I don't have Outlook 2003 to test with at the moment)

Solution

So, after some trial and error with a 2003 test system and I found that adding the following:

DTSTAMP:20101103T120000Z

allowed Outlook 2003 to import the .ics file without breaking 2007, iCal or Google.
I also, for good measure added the UID line to my final .ics file.

So, my final .ics file looks like this:

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 11.0 MIMEDIR//EN
VERSION:2.0
METHOD:PUBLISH
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
CLASS:PUBLIC
DESCRIPTION:Some description here
LOCATION:Scheduled PTO
DTEND;VALUE=DATE:20101104
DTSTART;VALUE=DATE:20101103
DTSTAMP:20101103120000Z
UID: VACATIOND41D8CD98F00B204E9800998ECF8427E
PRIORITY:5
SEQUENCE:0
SUMMARY;LANGUAGE=en-us:PTO - 8.00 hour(s)
X-MICROSOFT-CDO-ALLDAYEVENT:TRUE
X-MICROSOFT-MSNCALENDAR-ALLDAYEVENT:TRUE
END:VEVENT
END:VCALENDAR

All I did was create a manual entry in outlook 2003, saved it as a .ics and did a comparison on what was missing/different from my original.

All works well now.

UPDATE One additional change, I removed the VERSION:2.0 bit and that made all work even more reliably.