SharePoint sometimes displays dates in mm/dd/yyyy format despite the locale of the device being correctly specified.
This comes from the URL used to display the page. The locale is specified as part of the page URL.
If the locale (en-US) is specified in the URL, the dates display in alignment with this locale rather than the one set on the device.
Changing the locale in the URL alters the way the dates are displayed.
The following URLs navigate to the same site but display dates differently.
Changing the locale in the URL alters the way the dates are displayed.
The following URLs navigate to the same site but display dates differently.
Note that the first one shows dates in US format (mm/dd/yyyy) and the second in NZ format (dd/mm/yyyy).
https://tasks.office.com/hud.govt.nz/en-US/Home/Planner#/plantaskboard?groupId=e58d2b6d-d0ca-45ca-b39b-8767302a14c0&planId=mqptyhVlckqSRLASqPWJDMgAHRQf
https://tasks.office.com/hud.govt.nz/en-NZ/Home/Planner#/plantaskboard?groupId=e58d2b6d-d0ca-45ca-b39b-8767302a14c0&planId=mqptyhVlckqSRLASqPWJDMgAHRQf
https://tasks.office.com/hud.govt.nz/en-US/Home/Planner#/plantaskboard?groupId=e58d2b6d-d0ca-45ca-b39b-8767302a14c0&planId=mqptyhVlckqSRLASqPWJDMgAHRQf
https://tasks.office.com/hud.govt.nz/en-NZ/Home/Planner#/plantaskboard?groupId=e58d2b6d-d0ca-45ca-b39b-8767302a14c0&planId=mqptyhVlckqSRLASqPWJDMgAHRQf
To resolve the date issue, change the locale in the URL from en-US to en-NZ and reload the page.