[I caught some misconceptions here which I think will this post clear off eventually]
There should not be any difference since both refer to /var/run/utmp file, which has its own format to store the records. If at all there is any difference, then your utmp file is busted. uptime shows the amount of time that has passed since the system has been booted or how long the system has been running. It does not tell you the system clock or system boot time . System boot time information is stored /var/run/wtmp file.
[centos@centos temp]$ date; uptime; who -b
Fri Dec 9 20:41:40 IST 2011
20:41:40 up 1:32, 2 users, load average: 0.50, 0.37, 0.29
system boot 2011-12-09 19:11
uptime refers as well /proc/uptime, which essentially keeps the counters in kernel.
[centos@centos temp]$ sleep 1; cat /proc/uptime; uptime; sleep 5; cat /proc/uptime ; uptime
5914.79 5271.83
20:47:39 up 1:38, 2 users, load average: 0.29, 0.31, 0.27
5920.07 5276.80
20:47:44 up 1:38, 2 users, load average: 0.56, 0.36, 0.29
/var/run/wtmp is referred by last/lastb commands. who & w refers /var/run/utmp file. last reboot will show a log of all reboots since the log file was created.
Additionally, if you are having /proc filesystem, then tool such as procinfo can give you bootup time as well.
Example:
bash$ procinfo | grep Bootup
Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829
/var/log/messages
That is the main log file you should check for messages related to this. Additionally either /var/log/syslog (Ubuntu) or /var/log/secure (CentOS)
To find out when your server was last rebooted just type uptime to see how long it has been up.
Best Answer
uptimeIf you want it in numerical form, it's the first number in
/proc/uptime(in seconds), so the time of the last reboot isThe uptime includes the time spent in a low-power state (standby, suspension or hibernation).