timetag

Timetag is a label or value attached to an event to indicate when it should occur or when it occurred. In computing and communications, timetags provide precise timing information that supports scheduling, synchronization, and ordering of data across systems with clocks that may drift.

Formats vary by system. A common arrangement is a fixed-point timestamp consisting of an epoch reference and a time portion with a given resolution. For example, in the OSC (Open Sound Control) protocol, a timetag is a 64-bit fixed-point quantity: the high 32 bits encode the number of seconds since January 1, 1900, and the low 32 bits encode the fractional part, yielding sub-second precision. The value 0 is defined to mean immediately. Other contexts use Unix time (seconds since 1970) or other epochs, sometimes with separate fields for seconds and nanoseconds.

Usage examples include scheduling OSC messages at a future moment, timestamping sensor data for correlation, or ordering events in distributed systems where clocks are not perfectly synchronized. Timetags may be absolute, pointing to a fixed moment, or relative, indicating a delay from a reference time or from reception.

Implementation considerations include endianness, time zone independence, clock synchronization, and portability across platforms. Many libraries provide utilities to convert timetags to native system time, compare two timetags, or advance timetags by a duration. Handling of invalid or out-of-range timetags, clock skew, and leap seconds are common robustness concerns.

Timetag concepts intersect with timecode, timestamps, and synchronization protocols such as NTP and PTP.