The administrator can specify the date format by specifying it in the general preferences. The abbreviated date format appears on reports, forms, and data-entry pages.
This page is available at Setup > Company > Preferences > General Preferences
But individual user can override the date format specific to the account by specifying the same in the Set Preference
If the company and user select the different date formats, it will affect any scripts, workflow that referring to the company’s date format.
This can be solved by restricting the user to set date format and other features specific to accounts.
To prohibit individual overrides of company preferences
1. Go to Setup > Company > Preferences > General Preferences.
2. On the Overriding Preferences subtab, clear the Allow Override box next to the preferences you do not want to be overridden by individual user preferences.
3. Click Save.