timeofday

Time of day, or timeofday, refers to the portion of a 24-hour day as determined by civil or universal time, independent of the specific calendar date. It is the basis for daily activities, scheduling, and many technical processes. In practical terms, time of day is expressed in hours, minutes, and seconds, often with fractions for higher precision. Civil time uses local time zones to relate clock time to a reference such as Coordinated Universal Time (UTC). The relationship between local time and UTC can vary by location and may include daylight saving adjustments.

The day is commonly divided into segments such as morning, afternoon, evening, and night, with dawn and dusk marking transitions between light and dark. Fixed boundaries between these periods do not exist globally; cultural norms, seasons, and latitude influence how people categorize them.

In computing and data management, time of day is captured as a timestamp, typically representing seconds since a fixed epoch, often with subsecond precision. POSIX systems provide interfaces like gettimeofday to retrieve the current time of day as seconds and microseconds since the epoch, though some timezone fields are deprecated in modern usage. Timestamps are essential for logging events, scheduling tasks, measuring durations, and maintaining synchronization across systems.

Time of day is linked to broader time standards, including UTC, International Atomic Time (TAI), and the occasional insertion of leap seconds to keep civil time aligned with astronomical day length. The concept appears across disciplines—from astronomy and navigation to computer science and daily life—where it supports communication, coordination, and planning.