$conn_requeststhe sequence number of current request in the keepalive connection
$dollarThe sign of dollar.
$request_time_msecThe request processing time, which is in milliseconds, only used in log_format.
$request_time_usecThe request processing time, which is in microseconds, only used in log_format.
$unix_timeThe current timestamp, it's the current second since the epoch time of Unix.
$yearCurrent year, e.g. 2011
$year2Current year, e.g. 11
$monthCurrent month, e.g. 03, 12.
$dayCurrent day, e.g. 03, 22.
$hourCurrent hour (24), e.g. 02, 21.
$hour12Current hour (12), e.g. 02, 09.
$minuteCurrent minute, e.g. 35.
$secondCurrent second, e.g. 12.
$sent_cookie_XXXThe cookie value of XXX in the Set-Cookie headers
$host_commentHost and the timestamp, e.g.
“<!-- localhost Thu, 29 Dec 2011 10:10:56 GMT -->”.