Each user can choose in their preferences the date and time format used by MediaWiki when generating date and/or time, in page history, recent changes, etc.
The formats vary with the user's language. Options are made available for a language if listed in the $datePreferences array of its messages file (languages/messages/Messages*.php). The ID used in the list needs to be named in the $dateFormats array (whether the general one or the override in the Messages*.php file in question) and defined there with the appropriate sprintfDate() format; otherwise it will be ignored.
The options are then listed in Special:Preferences accordingly to the definitions, in the same order. They use January 15, 2001 as example. For English language (where d stands for day, m for month and y for year) they are:
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HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4