"The timestamps refer to the time the request was started, so it is possible to see log file entries that are out of order, especially if you are dealing with a mix of large files and small files being served."
I'm working with Apache 1.3.x right now and I've found that the timestamps refer to the time when the request is completed. Of course the log entry is created at the end because of the options to include the number of bytes sent and the time it took to complete. Of course the timestamp data could be captured at the beginning of the request anyway but that is not what I've found.
Of course this still means log entries will not always be in the order in which the files were requested.