We have had a few clients talk to us that the iCal sent from CiviCRM is incorrect. So we investigated this and we couldn't find any problems. When we opened the iCal in a Text Reader the time matched the event time and it entered into our calendars correctly. Must be a setting on the clients PC we thought, maybe incorrect Time Zone?
We checked Apple Calendars, Thunderbird, Google etc. All worked correctly. As you can see none of us use Windows or Outlook.....
So we thought, best check Outlook as well. So we booted up a PC, downloaded the iCal into Outlook and lo and behold.... Incorrect time!
Basically Outlook is looking at the time zone and adding on another hour when Daylight Savings is in place. No other calendar system is doing this. Multiple threads on time zones and iCal have been created by people trying to get Microsoft to change things but as always they are very slow to make any changes and really don't seem to care.
So at the moment the CiviCRM Community are trying to set up CiviCRM so it works with both Microsoft Outlook and all the rest of the calendars. Who knows, Microsoft might actually fix it at some point but we are not holding our breath on this.
Anyway there is more information here if you are interested in this:
CiviCRM Issue Log - https://lab.civicrm.org/dev/core/-/issues/2887
Potential Work Around
One option we can do is to edit the registration email template to remove the iCal link. This would mean that the email gets sent but no link to add it automatically to the calendar. We can also hide the iCal links on the sign up page using CSS.
If you would like us to implement this, then please raise a support ticket and we will action this for you.
This does demonstrate the ability of open source to work around issues when they arise and when the larger companies are slow to change.