Outlook Web Apps in provides option for the Office 365 users to change the display language, date and time format and current time zone. All these settings can be changed using the Outlook Web Apps Regional Settings.
Click the Option link available at the corner of the Outlook Web Apps and See All Options from the lit
In the Options page, Click on the Settings link available under the Account section.
Then click on the Regional tab
This would display the following regional settings.
To change the display language Outlook Web Apps, choose your preferred language for the Language drop down.
If you want to change the date and time format other than the language specific one then you can use Date format and Time format drop down respectively.
Similarly to set your time zone, you use the Current time zone drop down.
After making the required changes, click on the Save link available at the bottom of the page.