Finding login times in Linux

Question

I am trying to figure out times of logins into my systems (basically systems boot).

I am making use of last Unix command. However, it does not let me pull more than a certain number of entries. I assume that the log file from which it pulls, which is /var/log/wtmp, gets overwritten after a certain size.

I see that i have a wtmp.1 file also, so using -f parameter i can go back a month further back the logs using this parameter. Wondering if logs further back are archived somewhere.

So, my question is: Is there a way to get older entries.

The following is the last call that i am making:

last -n 10000|grep "system"

Here are last few lines of the output

reboot   system boot  3.5.0-36-generic Sun Jul  7 07:07 - 22:08  (15:01)    
reboot   system boot  3.5.0-36-generic Sat Jul  6 23:23 - 23:23  (00:00)    
reboot   system boot  3.5.0-34-generic Sat Jul  6 09:40 - 23:22  (13:42)    
reboot   system boot  3.5.0-34-generic Sat Jul  6 09:38 - 09:39  (00:00)    
reboot   system boot  3.5.0-34-generic Sat Jul  6 06:40 - 09:39  (02:58)    
reboot   system boot  3.5.0-34-generic Sat Jul  6 06:15 - 06:17  (00:02)    
reboot   system boot  3.5.0-34-generic Sat Jul  6 06:13 - 06:17  (00:03)    
reboot   system boot  3.5.0-34-generic Fri Jul  5 19:30 - 22:34  (03:03) 

I am not able to get logs further back in time.

1. Is this the correct approach?
2. How do we see older logs? For instance if i pass -n 10000 or -n 1000000, i get the same output.

Eventually i will write a quick Python script to parse this o/p from subprocess module.

EDIT : Most of the answers below are correct. Unfortunately could accept only one answer. The logs once gone are gone!

Answer 1

you don't say what type of unix / linux you are running but on my Ubuntu hosts this works good for last boot times

for f in /var/log/wtmp*; do last -f $f reboot;done

All it does is find all the wtmp files in /var/log and then filter out the reboot user

Answer 2

last searches back through the file /var/log/wtmp. So regarding 2) it can only list those entries contained in wtmp. (use parameter f to specify any other file) E.g. if you rotate that file with a log rotator, it won't see those entries per default. 1) depends ;-)

You can only list those logins for which the log (resp. the rotate log are still present)