The command to check the uptime is show version. The uptime can be found in the middle part of the output and it will look something like the below output. Moreover, if the router has an authoritative clock set, either via NTP or manual 'clock set' command, it will display the date/time when it was rebooted.
R2 uptime is 1 week, 2 hours, 12 minutes
System returned to ROM by reload
System restarted at 15:44:41 UTC Mon Oct 2 2006
System image file is "flash:c3640-js-mz.124-7.bin"