You can change your date/time format and your time-zone under Settings - Document settings - Local settings. This can be useful if you intend to send invoices with a specific date format or if you move from one country to another. The date format changes on the Billdu website and on invoices as well.
Your current date/time format and time-zone are based on your device setting. On iOS devices navigate to Settings - General - Language & Region - Region. You can see a date and number format as an example.
For a fast change to common date formats, navigate to your iOS Settings, locate the Billdu app, and find the Preferred language setting. Select different languages to change the date format of your invoices instantly (only iOS 13 and higher).
On Android devices, it can be found mostly at Settings - System. Please see the instructions for your mobile device in case you need to change some date/time or time-zone settings.
Note: The date format on invoices in your mobile app and online invoices (website) may differ because of different local settings.