MySQL 5.7 Release Notes
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.
The log_timestamps system
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 log_timestamps values
are UTC (the default) and
SYSTEM (the local system time zone).
Timestamps are written using ISO 8601 / RFC 3339 format:
YYYY-MM-DDThh:mm:ss.uuuuuuZ signifying Zulu time
(UTC) or ±hh:mm (an offset that
indicates the local system time zone adjustment relative to
UTC). For example:
2020-08-07T15:02:00.832521Z (UTC)
2020-08-07T10:02:00.832521-05:00 (SYSTEM)