$conn_requests the sequence number of current request in the keepalive connection.
$dollar The sign of dollar.
$request_time_msec The request processing time, which is in milliseconds, only used in log_format.
$request_time_usec The request processing time, which is in microseconds, only used in log_format.
$unix_time The current timestamp, it's the current second since the epoch time of Unix.
$year Current year, e.g. 2011.
$year2 Current year, e.g. 11.
$month Current month, e.g. 03, 12.
$day Current day, e.g. 03, 22.
$hour Current hour (24), e.g. 02, 21.
$hour12 Current hour (12), e.g. 02, 09.
$minute Current minute, e.g. 35.
$second Current second, e.g. 12.
$sent_cookie_XXX The cookie value of XXX in the Set-Cookie headers.
$host_comment Host and the timestamp, e.g.
“<!-- localhost Thu, 29 Dec 2011 10:10:56 GMT -->”.
$ssl_handshake_time used for monitoring SSL handshake time.