convert date and time to epoch in awk

[b w]

I want to write a script that parses the last, say, 10 minutes of a log
file looking for a certain string, like 'error', or failed', and returns
how many times it shows up. The script would be run by Nagios and if it
returns > 0 an alert is raised. Each line of the log file starts with a
date like 'Feb 19 23:45:32'.

One way to do it I guess would be to read each line in a while loop,
extract the date, convert it into epoch using the date command, if it's
within 10 minutes remember the line somewhere, then grep the result. I was
thinking this might be too slow, or there may be too many lines at some
point, but it might actually be acceptable if I tail the last few thousands
lines. Anyway...

Another way would be to use gawk, which has date/time functions like
systime() and mktime(). This works fine, but someone like myself at some
point will forget to install gawk on a new server and might not realize it
untill something happens.

So, is there a way to compare two dates in FreeBSD's awk or convert a date
to epoch? Or some other fast way to select the last 10 minutes from a log
file? An example would be appreciated, if possible.