Most lines in the logfile start with the date the log entry was created, like
Fri Sep 9 03:41:07 [conn1618300] query [...] reslen:64 829ms
Unfortunately, sometime, there are also entries, which don't follow this convention, such as

2300/22660 10%
12000/22660 52%
19800/22660 87%
100/103 97%
3300/10050 32%
8700/23255 37%
19500/23255 83%
800/2111 37%
unindex failed (key too big?)

and some more.

The presence of these lines makes it way more difficult to parse the log file for important entries (with logwatch for example) ...