If you want to know how long a Mac OS X system has been running since the last reboot, you can use the uptime command, which can be run from a command line interface available from a Terminal window.
$ uptime 22:17 up 20 days, 12:24, 18 users, load averages: 2.95 2.93 2.46 $
The output above shows that the time is 10:17 PM (the time is in 24-hour clock format, often referred to as military time) and that it has been 20 days, 12 hours, and 24 minutes since the system was last rebooted. The three numbers at the end of the output, 2.95, 2.93, and 2.46, are the load average of the system over the last 1, 5, and 15 minutes.
The output above shows 18 users on the system when the uptime command was run. If you want to see a list of those users, use the w command. If you are the only user of the system, don't worry that you may see a number greater than 1 listed; it's likely they are all logins for your account as they were when I checked for the above instance where "18 users" was listed. The output is showing every logged in session and in this case I had multiple shell prompts through open tabs in the Terminal application.