Timestamp

A timestamp is a sequence of characters or encoded information identifying when a certain event occurred, usually giving date and time of day, sometimes accurate to a small fraction of a second. Timestamps do not have to be based on some absolute notion of time, however. They can have any epoch, can be relative to any arbitrary time, such as the power-on time of a system, or to some arbitrary time in the past.

The term "timestamp" derives from rubber stamps used in offices to stamp the current date, and sometimes time, in ink on paper documents, to record when the document was received. Common examples of this type of timestamp are a postmark on a letter or the "in" and "out" times on a time card.

In modern times usage of the term has expanded to refer to digital date and time information attached to digital data. For example, computer files contain timestamps that tell when the file was last modified, and digital cameras add timestamps to the pictures they take, recording the date and time the picture was taken.

Digital timestamps

This data is usually presented in a consistent format, allowing for easy comparison of two different records and tracking progress over time; the practice of recording timestamps in a consistent manner along with the actual data is called timestamping.

Timestamps are typically used for logging events or in a sequence of events (SOE), in which case each event in the log or SOE is marked with a timestamp.

Practically all computer file systems store one or more timestamps in the per-file metadata.
In particular, most modern operating systems support the POSIX stat (system call), so each file has three timestamps associated with it:
time of last access (atime: ls -lu),
time of last modification (mtime: ls -l), and
time of last status change (ctime: ls -lc).

Some file archivers and some version control software