TIMESTAMP

A timestamp. The range is '1970-01-01 00:00:01' UTC to '2038-01-19
03:14:07' UTC. TIMESTAMP values are stored as the number of seconds
since the epoch ('1970-01-01 00:00:00' UTC). A TIMESTAMP cannot
represent the value '1970-01-01 00:00:00' because that is equivalent to
0 seconds from the epoch and the value 0 is reserved for representing
'0000-00-00 00:00:00', the "zero" TIMESTAMP value.

Unless specified otherwise, the first TIMESTAMP column in a table is
defined to be automatically set to the date and time of the most recent
modification if not explicitly assigned a value. This makes TIMESTAMP
useful for recording the timestamp of an INSERT or UPDATE operation.
You can also set any TIMESTAMP column to the current date and time by
assigning it a NULL value, unless it has been defined with the NULL
attribute to permit NULL values. The automatic initialization and
updating to the current date and time can be specified using DEFAULT
CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses, as described
in
http://dev.mysql.com/doc/refman/5.5/en/timestamp-initialization.html.

URL: http://dev.mysql.com/doc/refman/5.5/en/date-and-time-type-overview.html

