Apache logs: Get unique visitors in the last 60 seconds

This is modified from: http://nixcraft.com/showthread.php/13554-Script-to-count-unique-ips-in-apache-access-log/page3?p=32816&viewfull=1#post32816

To get the number of unique visitors (by ip) in the last 60 seconds from an Apache log:

awk  '{if ( substr($4,14) > strftime("%H:%M:%S", systime()-(60) )  && substr($4,2,9) == strftime("%d/%b/%C")) !a[$1]++}END{for(i in  a) k++; print k }' access_log