$uptime(system, 1) is a variable in mIRC
scripting that, when said in a chatroom, will display the user's uptime in yrs/mths/wks/days/hrs/mins/secs.
Most people have their uptime script display something along the lines of "My Computer Has Been Online for (insert uptime here)." This can easily by done in the Alias section of mIRC by creating a new line:
/utime /say (or /echo if you don't want anybody to hear it) My Computer Has Been Online for $uptime(system, 1)
After doing this, all you need to do is type /utime in a room, status window, or message window, and you can proudly tell anyone you want to how long your computer has been running since you last rebooted.