If you need to know how long it's been since a Mac rebooted, uptme is the command for you. It shows the current time of day, plus the elapsed time since last reboot in days, hours, minutes, and seconds. For some reason, Windows doesn't have this command, but it probably isn't up long enough to matter.
Also, uptme shows the number of users logged in and the load averages (the number of processes waiting to run) of the system over the last one, five, and 15 minutes. What values are good or bad for load average depend on the number of CPUs available. A load average of 4 isn't bad for a quad-core Mac, but it would indicate a very busy single-core system.
16:04 up 721 days, 15:37, 2 users, load averages: 0.72 0.81 0.81
These are the 20 OS X command-line utilities you can get the most value from. Now it's up to you to do so!