A string unique to a time duration of 24 hours between 2 successive midnights defined by the local time zone. The specific representation of a date will depend on which calendar convention is in force; e.g., Gregorian, Islamic, Japanese, Chinese, Hebrew etc. as well as local ordering conventions such as UK: day/month/year, US: month/day/year.
Inputting and outputting dates on computers is greatly complicated by these localisation issues which is why they tend to operate on dates internally in some unified form such as seconds past midnight at the start of the first of January 1970.
Unix manual page: date(1), ctime(3).