Civil day

From Glossary of Meteorology

civil day

A mean solar day reckoned from midnight to midnight, usually divided into two 12- hour divisions, although now often regarded as a single 24-hour series.

As practically applied by use of meridional time zones of approximately 15° width, civil time is accurate (relative to mean solar time) to within ±30 minutes, or 7½° of longitude. Minor exceptions occur when the time zones have been established according to geopolitical considerations, rather than adhering strictly to the meridians.
See apparent solar day, day.