Opening .ics calendar invites causes the event to be registered an hour late

You can edit the ICS manually, or you can ask the sender to reconfigure or update their configuration, particularly if they’re in the Eastern US.

Anything that’s confused enough to be sending out Eastern Standard Time for a September date either needs some configuration help (in the eastern US), or they’re in a US timezone I’m unaware of, or they’re in Quintana Roo, Panama, Colombia, Peru, Ecuador, or probably a few other places that sty on EST all year, and the schedule shift you are seeing for those is correct for those places..

If you’re getting these often and if the sender is recalcitrant, automating the ICS file fix is feasible.

What’s in the .ics file, if it can be opened somewhere (else) that can peek into the .ics?

Might be able to save it from Mail to Files, then rename it to something.txt, and peek into it, but I don’t have an .ics handy to check that.

Here’s an example from Wikipedia:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//hacksw/handcal//NONSGML v1.0//EN
BEGIN:VEVENT
UID:uid1@example.com
DTSTAMP:19970714T170000Z
ORGANIZER;CN=John Doe:MAILTO:john.doe@example.com
DTSTART:19970714T170000Z
DTEND:19970715T040000Z
SUMMARY:Bastille Day Party
GEO:48.85299;2.36885
END:VEVENT
END:VCALENDAR

DTSTART and DTEND are the start and end and typically in the UTC (Zulu) timezone.

If you save the following slightly-changed-from-your-posting .ics file somewhere, and then use Finder > Quick Look (or via the ⌘Y shortcut) to view the file, do you get an event from 12:00 to 13:00 on September 9th 2023?

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 15.0 MIMEDIR//EN
VERSION:2.0
METHOD:PUBLISH
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
CLASS:PUBLIC
CREATED:20230822T222954
DESCRIPTION:n09/09/2023\n12:00 PM
DTEND;TZID="Eastern Standard Time":20230909T130000
DTSTART;TZID="Eastern Standard Time":20230909T120000
SUMMARY:9/9/23
TRANSP:TRANSPARENT
BEGIN:VALARM
TRIGGER:-PT1440M
ACTION:DISPLAY
DESCRIPTION:Reminder
END:VALARM
END:VEVENT
END:VCALENDAR

With Finder > Quick Look, I get this:

It’s currently EDT. Not EST.

Apple seems to try to convert EST to EDT here.

Whether that conversion is the appropriate behavior, or reporting an error for an inconsistent time in the ICS is the right answer?

What to do? Check the time and timezone on the sender’s computer.

LynxV1 wrote:

What was slightly changed? Copy pasta'd the codes and they're the same?

They're not identical. Having spent a fair chunk of my time looking at this stuff and researching the details of ical ics files for this answer particularly around the TZID string, and now spending time with diff and sdiff to show the (slight) differences, and then thought, well, why am I spending time proving a file that is slightly different is different?

I'd wondered whether the issue was arising with the timezone name (US/Eastern would be typical—Microsoft seems to have their own interpretation of TZID too) within the calendar parser processing in whatever app was parsing the ics file, or if it was also arising within Quick Look. This because off-by-an-hour issue with the demo file is not arising in macOS 13 and Quick Look here, so I'm left to wonder what is different in your particular environment.

Microsoft tooling is clearly involved, and which makes fully reproducing this ics file issue locally not an option.