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. The term 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. A common example of this type of timestamp is a postmark on a letter. However, 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.
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A timestamp is the current time of an event that is recorded by a computer.
Timestamps are employed extensively within computers and over networks for various types of synchronization. For example, they are assigned to packets in some network protocols in order to facilitate the reassembly of the data (e.g., human speech) in the proper sequence by the receiving host (i.e., computer). Also, they are used by database management systems (DBMS) to determine the transaction order in the event of a system failure (e.g., a computer crash caused by a loss of electrical power or disk failure).
Timestamps are also routinely used to provide information about files, including when they were created and last accessed or modified. This information is included in the inode, which is a data structure on a filesystem on a Unix-like operating system that stores all the information about a file except its name and its actual data.
Another important application is events that are recorded in system log files. The timestamps in such files can be extremely useful for monitoring system security and for forensic purposes.
The time as recorded by timestamps can be measured in terms of the time of day or relative to some starting point. And it is measured with high precision in small fractions of a second.
The accuracy of the time is maintained through a variety of mechanisms, including the high-precision clocks built into computers and the network time protocol (NTP). NTP uses coordinated universal time (UTC) to synchronize computer clock times to a millisecond (and sometimes to a fraction of a millisecond) and uses UDP (user datagram protocol), one of the core Internet protocols, as its transport mechanism.
sources: http://www.linfo.org/timestamp.html
http://en.wikipedia.org/wiki/Timestamp
