What timezone has been set up for computer? Please post an image that shows this.

In Settings > Calendar If you select 'Set timezone manually' - please post image showing the selected timezone.

If it was set as 'Use system timezone' and your computer has the ability to auto set timezones then if you were travelling through different timezones it would be updated.

The 17 hour difference explains why it moves forward to the next day, but that implies different timezones.

Where are you located? Do you travel with computer ?

re :Someone sends me a calendar appointment Select email and clickon 'More' > 'View Source' What is in the 'Date' in the top 'Received by server' eg: Thu, 8 May 2025 14:33:26 +0100 What is the date in the actual 'Date' header: eg: Thu, 8 May 2025 13:33:24 +0000

See attached Thunderbird Calendar Settings + my GNOME System Timezone settings.

Australia/Sydney is my timezone. I haven't left my timezone for more than a year.

My most recent calendar invite should be 11th of May, 12:00-14:30, but is currently scheduled for 11th of May, 22:00 to 12th of May, 00:30.

(not 17 hours; I misremembered, looks like 10 instead)

As for the Source:

The Received field:

Received: ; Wed, 7 May 2025 18:53:27 +1000

The Date field:

Date: Wed, 07 May 2025 18:53:25 +1000

What are the results of checking the TZ variable in bash. The GUI timezone information on some Linux systems does not relate to the system, only the GUI component. Fairly typical of Linux really. Multiple ways to do everything and nothing simple can be relied upon to actually be a universal setting. It is really confusing for folk like me that really don't use anything but windows

Linux has no default timezone and therefore time is UTC. Windows on the other hand defaults to US pacific time UTC-7 Your timezone is UTC +10 hours and your appointments are reverting to UTC. That sounds a lot to me like you will find the TZ variable to actually be blank.

Try some of the methods shown here https://www.cyberciti.biz/faq/find-checking-timezone-in-linux-using-command-line/ I really do not know if you can rely in the gnome GUI in this instance. So I err on the side of caution and assume you can't.

I set the timezone myself when I installed the system by symlinking my zoneinfo to /etc/localtime, and this works correctly when I run the date command.

Yes, my TZ variable is blank.

The ArchWiki has some info on how the TZ variable can be used for temporarily getting an application to see a new timezone, which works with the date command, so I decided to try it with Thunderbird:

--- $ TZ=":/usr/share/zoneinfo/Australia/Sydney" thunderbird console.error: (new UnknownCollectionError("Unknown Collection \"thunderbird/url-parser-default-unknown-schemes-interventions\"", "resource://services-settings/RemoteSettingsClient.sys.mjs", 189)) console.error: (new UnknownCollectionError("Unknown Collection \"thunderbird/moz-essential-domain-fallbacks\"", "resource://services-settings/RemoteSettingsClient.sys.mjs", 189)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) ---

My appointment times were still screwed up when I did this. But it is interesting that there is a Calendar error (which appears regardless of whether I set the TZ variable when running Thunderbird).

Running timedatectl provides another hint that matches up with what you're saying about my appointments reverting to UTC:

--- $ timedatectl

              Local time: Fri 2025-05-09 12:29:06 AEST
          Universal time: Fri 2025-05-09 02:29:06 UTC
                RTC time: Fri 2025-05-09 02:29:06
               Time zone: Australia/Sydney (AEST, +1000)

System clock synchronized: yes

             NTP service: active
         RTC in local TZ: no

---

My universal time is 10 hours behind my local time. Although my appointments are actually 10 hours ahead of where they should be...

I can reproduce this issue across two computers, and one of them is a single-boot Archlinux while the other is a dual-boot with Windows 10; when i boot into Windows, the time will always be wrong. Same timedatectl output.

Running timedatectl with TZ set to ":/usr/share/zoneinfo/Australia/Sydney" doesn't change the output.

Not sure what else I can try here.

The bizarre thing is that no other application on my computer seems to care about my Universal Time and behaves just fine; only Thunderbird is acting up. And not even reliably!

At first, when I accept the appointments, they are set for the correct time. It's only when I restart Thunderbird that the appointments shift time. And I had to restart it twice for Thunderbird to screw up the times for both appointments; it only screwed up one at a time.

I set the timezone myself when I installed the system by symlinking my zoneinfo to /etc/localtime, and this works correctly when I run the date command.

Yes, my TZ variable is blank.

The ArchWiki has some info on how the TZ variable can be used for temporarily getting an application to see a new timezone, which works with the date command, so I decided to try it with Thunderbird:

--- $ TZ=":/usr/share/zoneinfo/Australia/Sydney" thunderbird console.error: (new UnknownCollectionError("Unknown Collection \"thunderbird/url-parser-default-unknown-schemes-interventions\"", "resource://services-settings/RemoteSettingsClient.sys.mjs", 189)) console.error: (new UnknownCollectionError("Unknown Collection \"thunderbird/moz-essential-domain-fallbacks\"", "resource://services-settings/RemoteSettingsClient.sys.mjs", 189)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) console.warn: Calendar: (new Error("Property X-ALT-DESC not set", "resource:///modules/CalItemBase.sys.mjs", 550)) ---

My appointment times were still screwed up when I did this. But it is interesting that there is a Calendar error (which appears regardless of whether I set the TZ variable when running Thunderbird).

Running timedatectl provides another hint that matches up with what you're saying about my appointments reverting to UTC:

--- $ timedatectl

              Local time: Fri 2025-05-09 12:29:06 AEST
          Universal time: Fri 2025-05-09 02:29:06 UTC
                RTC time: Fri 2025-05-09 02:29:06
               Time zone: Australia/Sydney (AEST, +1000)

System clock synchronized: yes

             NTP service: active
         RTC in local TZ: no

---

My universal time is 10 hours behind my local time. Although my appointments are actually 10 hours ahead of where they should be...

I can reproduce this issue across two computers, and one of them is a single-boot Archlinux while the other is a dual-boot with Windows 10; when i boot into Windows, the time will always be wrong. Same timedatectl output.

Running timedatectl with TZ set to ":/usr/share/zoneinfo/Australia/Sydney" doesn't change the output.

Not sure what else I can try here.

The bizarre thing is that no other application on my computer seems to care about my Universal Time and behaves just fine; only Thunderbird is acting up. And not even reliably!

At first, when I accept the appointments, they are set for the correct time. It's only when I restart Thunderbird that the appointments shift time. And I had to restart it twice for Thunderbird to screw up the times for both appointments; it only screwed up one at a time.

re :$ TZ=":/usr/share/zoneinfo/Australia/Sydney" thunderbird

Have you tried "Australia/AEST" or "Australia/Melbourne" to see if it works ?

Hey, TZ=Australia/AEST actually works!

Rather than setting TZ in any of my dotfiles globally for my user, I've set Exec=env TZ=Australia/AEST thunderbird in my .desktop file and done the same thing in my keyboard shortcut for Thunderbird.

And it works.

I have no idea why Thunderbird ignores me manually setting my timezone to Australia/Sydney in the Calendar settings, but at least this works.

Thanks so much!

Many thanks for you feedback. I'm sure there could be others who may experience the same problem. Glad to hear all is now sorted.