You could get it from /proc/version, but that should be the same stamp you'd get from 'uname -v'
Code:
$ grep -Eo '[a-zA-Z]{3}.[a-zA-Z]{3}.[0-9]{1,2}.[0-9]{2}:[0-9]{2}:[0-9]{2}.[a-zA-Z]{3}.[0-9]{4}' /proc/version
Tue Jun 10 11:56:48 CDT 2008
$ uname -a | grep -Eo '[a-zA-Z]{3}.[a-zA-Z]{3}.[0-9]{1,2}.[0-9]{2}:[0-9]{2}:[0-9]{2}.[a-zA-Z]{3}.[0-9]{4}'
Tue Jun 10 11:56:48 CDT 2008
$ uname -v | grep -Eo '[a-zA-Z]{3}.[a-zA-Z]{3}.[0-9]{1,2}.[0-9]{2}:[0-9]{2}:[0-9]{2}.[a-zA-Z]{3}.[0-9]{4}'
Tue Jun 10 11:56:48 CDT 2008
I'm sure that regex could be simplified... I'm NOT sure that it will get every possible time stamp... But you can see from here that uname and /proc/version should both have the same date for the kernel build, and this should be the accurate kernel build date.
Hope this helps!
-J