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 settings. On iOS devices navigate to Settings - General - Language & Region - Region. You can see a date and number format example
On Android devices it can be found mostly at Settings - System. Please see instructions for your mobile device in case you need to change some date/time or timezone settings.
The date format of your online invoices and invoices from your mobile device may differ because of different settings or mobile device capabilities.