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.

An actual time-stamp from around 1960.

A distinction is sometimes made between the terms datestamp, timestamp and date-timestamp:

Datestamp or DS: A date, for example 2024-11-18 according to ISO 8601
Timestamp or TS: A time of day, for example 06:09:39 using 24-hour clock
Date-timestamp or DTS: Date and time, for example 2024-11-18, 06:09:39

History

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.

With the advent of digital data systems, 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.^[1]

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, when they copy a file from some remote computer to the local computer, adjust the timestamps of the local file to show the date/time in the past when that file was created or modified on that remote computer, rather than the date/time when that file was copied to the local computer.

Timestamps are often found to be dirty in many cases. Without cleaning up inaccurate timestamps, time-related applications such as provenance analysis or pattern queries are not reliable. To evaluate the correctness of timestamps, temporal constraints can be applied, declaring distance limits between timestamps.^[2]

Standardization

ISO 8601 standardizes the representation of dates and times.^[3] These standard representations are often used to construct timestamp values.

Examples

Examples of date-timestamps:

Thurs 12/31/2009 1:35 p.m. — mixed-endian date, big-endian 12-hour clock
Thurs 31.12.2009 13:35 — same time as the above, different format with little-endian date and big-endian 24-hour clock
2005-10-30 T 10:45 UTC — ISO 8601 international order (big-endian) with time zone)
2007-11-09 T 11:20 UTC — same format as the above, hence easy to compare and perform alphanumeric sorting
Sat Jul 23 02:16:57 2005
2009-10-31T01:48:52Z — ISO 8601
2009-10-31 01:48:52Z — "Internet time" per RFC 3339, based on ISO 8601
1256953732 — Unix time, equivalent to 2009-10-31 T 01:48:52Z
1969-07-21 T 02:56 UTC
07:38, 11 December 2012 (UTC)
1985-102 T 10:15 UTC — year 1985, day number 102, i.e., 1985 April 12
1985-W15-5 T 10:15 UTC — year 1985, week number 15, weekday 5 = 1985 April 12
20180203073000 — used in Wayback Machine memento URLs, equals 3 February 2018 07:30:00

Examples of datestamps:

2025-05-25 — ISO 8601 international representation of 2025 May 25

Examples of timestamps:

17:30:23 — time of day in an afternoon
123478382 ns — the number of nanoseconds since boot
17 minutes — an arbitrary minute counter that increments every 1 minute since its last manual "reset" event

Sequence number:

21 — a unitless counter that indicates only the relative order of events; this is event #21, which comes after 20 and before 22

References

^ Claudia Maria Bauzer Medeiros (19 September 2009). ADVANCED GEOGRAPHIC INFORMATION SYSTEMS -Volume I. EOLSS Publications. p. 59. ISBN 978-1-905839-91-9.
^ Song, Shaoxu; Huang, Ruihong; Cao, Yue; Wang, Jianmin (May 2021). "Cleaning timestamps with temporal constraints". The VLDB Journal. 30 (3): 425–446. doi:10.1007/s00778-020-00641-6. ISSN 1066-8888. S2CID 7559769.
^ "ISO 8601:2004(E)" (PDF). ISO. 1 December 2004. Retrieved 7 March 2010. 3.5 Expansion … By mutual agreement of the partners in information interchange, it is permitted to expand the component identifying the calendar year, which is otherwise limited to four digits. This enables reference to dates and times in calendar years outside the range supported by complete representations, i.e. before the start of the year [0000] or after the end of the year [9999].

[Medeiros2009-1] Claudia Maria Bauzer Medeiros (19 September 2009). ADVANCED GEOGRAPHIC INFORMATION SYSTEMS -Volume I. EOLSS Publications. p. 59. ISBN 978-1-905839-91-9.

[2] Song, Shaoxu; Huang, Ruihong; Cao, Yue; Wang, Jianmin (May 2021). "Cleaning timestamps with temporal constraints". The VLDB Journal. 30 (3): 425–446. doi:10.1007/s00778-020-00641-6. ISSN 1066-8888. S2CID 7559769.

[3] "ISO 8601:2004(E)" (PDF). ISO. 1 December 2004. Retrieved 7 March 2010. 3.5 Expansion … By mutual agreement of the partners in information interchange, it is permitted to expand the component identifying the calendar year, which is otherwise limited to four digits. This enables reference to dates and times in calendar years outside the range supported by complete representations, i.e. before the start of the year [0000] or after the end of the year [9999].

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