Uptime is a measure of the time a computer system has been "up" and running. It came into use to describe the opposite of downtime, times when a system was not operational. The uptime and reliability of computer and communications facilities is sometimes measured in nines (similar to the unit of metallic purity). "Five nines" means 99.999% availability, which translates to a total downtime of approximately five minutes and fifteen seconds per year.

It is often used as a measure of computer operating system reliability and stability, in that this time represents the time a computer can be left unattended without crashing, or needing to be rebooted for administrative or maintenance purposes. Conversely, long uptime can indicate negligence, because critical updates can sometimes require reboots.

Records and comparisons

The Uptime-Project, collected data on uptimes from users until 1 March 2007, and the current record for longest uptime is 11 years, 303 days, 20 hours and 57 minutes on a computer running OpenVMS. Rumours mention in January 2008 that Irish Rail had an OpenVMS machine up for 18 years, and was restarted just for Y2K tests.

Netcraft maintains the uptime records for many thousands of web hosting computers.

The uptime of a personal computer is sometimes displayed as a badge of honour on an email signature or web site/forum. This was especially true in the Windows 9x daysFact: date=June 2008, where Windows NT and Windows 2000 users would boast of uptimes of more than 30 days, whereas many real-world Windows 9x installations crashed more often. In more recent times very long uptimes for home users with Windows NT and Windows 2000 machines are less striking because the Windows 9x line has been replaced by the Windows NT-based Windows XP.

Determining system uptime

Users of Windows XP Professional, Windows Server 2003 and Windows Vista systems can type systeminfo at the Command Prompt to display all system information, including the System Up Time.

C:\> systeminfo | find "Up Time" System Up Time: 0 Days, 8 Hours, 7 Minutes, 19 Seconds

Users of Unix-like systems can use the uptime(1) utility to get the uptime, together with the current time, the number of users and load averages for the past 1, 5 and 15 minute intervals:

$ uptime 01:98:37 up 2287 days, 2:52, 25 users, load average: 1.76, 1.26, 0.70

Users of OpenVMS systems can type show system at the command prompt:

$ show system OpenVMS V7.3-2 on node JERRY 29-JAN-2008 16:32:04.67 Uptime 894 22:28:52

Showing the uptime as days then hours:minutes:seconds

See also

• Myth of the nines for a discussion of downtime and availability
• High availability
• Downtime
• Maintenance window
• List of Unix programs
• Reliability disambiguation page
• Website monitoring