This is actually not a template-part, it's a setting. Setting it allows the date and time preferences when to be localized. Names of months and days will be in the desired language, etc.
The possible values depend according to which machine Nucleus is running on. Some possible values are
More info in the Open Group Specification for strftime. (opens a new window)
The locale is used for the date and time format, for the dateheaders, and for the archivelist items