Date/time format

Dear @Harald-René Flasch, 

Thank you for sharing your concern.  

This behavior occurs because different Microsoft 365 components read regional, language, and time settings from different sources rather than from a single global configuration. Below are the steps that can help verify and align those settings as much as possible. 

Notes: 

• Please note that some of these steps were already mentioned earlier by Q&A Assist. I am including them again here, along with screenshots and brief instructions, to ensure everything is configured correctly. 
• Since your post is tagged for the new Outlook app, the instructions above reflect that version. If you are using classic Outlook instead, feel free to tell me and I will provide the exact steps.  
• Before starting, I recommend ensuring that Windows, Microsoft 365 apps and your web browser are fully updated. 

Step 1: Confirm Windows time format 

I understand that you have already completed this step. The screenshot below is provided only to help confirm that the settings are configured correctly. 

Step 2: Verify Calendar settings in Outlook on the web 

Open Outlook on the web and switch to Calendar. 

Open Settings, then confirm: 

• First day of the week is set to Monday.

• Under General > Language and time, the region, date format, and time format are set correctly. 

These settings influence Calendar behavior and some task related views. 

Step 3: Align Microsoft 365 account language and time zone 

Some Microsoft 365 web apps read settings directly from the Microsoft account profile. 

Open an InPrivate or Incognito browser window. 
Sign in at https://www.office.com using the affected account. 

Select Apps > Choose open Word > Settings > Under Language and time zone, choose "Change your language". 

Next, confirm that the region and date and time format are set correctly. 

Step 4: Check Excel for the web regional settings 

The Date Picker format depends on both per user settings and the SharePoint site where the file is stored. 

Open Excel for the web. 

Open a file. 

Go to File > Options > Regional Settings. 

Confirm the regional format. 

This setting overrides the SharePoint site regional settings for your session. The preference is stored using a third-party cookie. 

Note: If third party cookies are blocked in the browser, Excel for the web cannot remember this preference and may revert to the default format. 

Next, you can set the preferred regional format in Excel for the web:

• After changing settings: 

If any Microsoft 365 language or time zone settings were updated, it can take up to 24 hours for all web apps to fully reflect the changes. 

If the old format still appears after waiting, you can try clearing the cache in your browser or desktop apps: 

1/ For New Outlook desktop app:  

Close the Outlook app completely.     

Open the Run dialog by pressing Windows + R.     

Type or paste the following command and click OK:     

olk.exe --devtools     

The new Outlook will open (you may need to sign in), and the DevTools window will appear.     

In DevTools:      

Click Application on the toolbar.     

Clear local storage cache: Under Storage, find https://outlook.office.com under Local Storage, right-click it, and select Clear.     

Clear cookies: Under Cookies, locate https://outlook.office.com., right-click it, and select Clear.     

Close the DevTools window.     

Once completed, the cache for the new Outlook will be cleared. Please note that this process removes temporary files, including saved login credentials, so you may need to sign in again.  

2/ For web apps 
Open your browser settings and clear cached data. 
Select the option to clear data for all time to ensure no outdated preferences remain. 

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I hope this information is helpful. Please feel free to let me know whether you are using the new Outlook for desktop, Outlook on the web, or classic Outlook and share the outcome after completing the steps above. 

Thank you for your patience and understanding. 

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Outlook and Excel do not each have a single global setting for date/time; different components read settings from different places. To get as close as possible to a consistent 24‑hour clock and Monday as first day of week, adjust both Windows and the Microsoft 365 account/individual app settings where available.

1. Confirm Windows regional settings

• In Windows, ensure the region/format is set to one that uses 24‑hour time and Monday as first day of week.
• Outlook for Windows uses the computer’s country/region settings to determine how times appear in Calendar and other views. Changing this is done in the operating system, not in Outlook itself.

2. Adjust Microsoft 365 language/region settings (for web apps) Some web experiences (Outlook on the web, Excel for the web, other Microsoft 365 web apps) use the account’s language/region settings rather than Windows:

• Open the Microsoft 365 account’s personal language and region settings.
• In the Language and Region settings table:
  • Set Locale to the desired language/region.
  • Under Time Format, choose 24 hour.
  • Under Set Your Calendar, choose the calendar type and ensure Define Your Work Week has Monday as a workday and that the calendar is configured with Monday as the first day of the week. These settings control how many Microsoft 365 web apps display dates, times, and the first day of the week.

3. Excel for the web regional format Excel for the web can override the SharePoint/site regional settings per user and per browser session:

• In Excel for the web, select File > Options > Regional Settings.
• Choose a region that uses 24‑hour time and Monday as first day of week, then select Change.
• Alternatively, from Home > Number Format > More number formats…, under Date, Time, or Special:
  • Pick the desired Locale.
  • Select Apply this locale as default, then OK. This affects how the date picker and date/time formats appear in Excel for the web for that user in that browser (stored via third‑party cookie).

4. Outlook Calendar vs Tasks

• Outlook Calendar’s 12/24‑hour display is driven by Windows regional settings.
• Tasks and some other components may still rely on a mix of Windows and account/app‑specific regional settings. Ensure both Windows and Microsoft 365 language/region settings are aligned as above.

5. Web apps showing 12h/Sunday

• For Microsoft 365 web apps, verify that the personal Language and Region settings are set to a locale that uses 24‑hour time and Monday as first day of week, and that Time Format is explicitly set to 24 hour.
• For Excel for the web, also ensure the workbook’s regional format (step 3) matches.

If any app still shows inconsistent behavior after these changes, it is typically due to that app not fully honoring all regional options yet; in those cases, aligning both Windows and Microsoft 365 account regional settings is the maximum available configuration.

References:

• Switch Calendar views between a 12-hour clock and 24-hour clock
• Change regional format settings in Excel for the web
• Change your personal language and region settings
• Using Date and Time Picker Controls
• Date and Time Picker Control Overviews