Configure Thunderbird to display ISO time stamps by setting up the Swedish locale on Debian/Ubuntu systems.

The Problem

Thunderbird doesn’t natively support ISO time stamp formatting, which can be frustrating for users who prefer standardized time representations. This is a known issue tracked in Mozilla’s bugzilla.

The Solution

Set up the Swedish locale (en_SE) which uses ISO date formatting:

sudo ln -s /usr/share/i18n/locales/en_DK /usr/share/i18n/locales/en_SE

Step 2: Add Locale to Generation List

echo 'en_SE.UTF-8 UTF-8' | sudo tee -a /etc/locale.gen

Step 3: Generate the Locale

sudo locale-gen

Step 4: Set System Time Locale

echo 'LC_TIME=en_SE.UTF-8' >> /etc/default/locale

Step 5: Configure Thunderbird (Optional)

In Thunderbird’s advanced configuration:

  1. Go to Preferences → Advanced → Config Editor
  2. Add a new string preference: intl.locale.requested
  3. Set the value to: en-SE

Why This Works

  • Swedish locale: Sweden uses ISO 8601 date format (YYYY-MM-DD) as standard
  • English base: en_SE provides English language with Swedish formatting
  • System-wide: The locale setting affects all applications, not just Thunderbird
  • Thunderbird integration: The optional config ensures Thunderbird respects the locale

Verification

After restarting Thunderbird, you should see timestamps in ISO format:

  • Instead of: 12/31/2023 11:59 PM
  • You’ll see: 2023-12-31 23:59

Background Information

This issue has been discussed extensively in the Mozilla community. For detailed technical background, see:

Alternative Approaches

If you prefer not to modify system locales, you can:

  1. Use environment variables when launching Thunderbird:

    LC_TIME=en_SE.UTF-8 thunderbird

  2. Create a wrapper script that sets the locale before launching Thunderbird

  3. Use a different locale that supports ISO formatting (like en_DK)

The system-wide approach described above is the most permanent and reliable solution for consistent ISO time stamp display across all applications.