The ID included in error log messages is that of the thread within mysqld responsible for writing the message. This indicates which part of the server produced the message, and is consistent with general query log and slow query log messages, which include the connection thread ID.
variable controls the time zone of timestamps in messages
written to the error log (as well as to general query log and
slow query log files). Permitted values are
UTC (the default) and
SYSTEM (local system time zone).