I sometimes noticed for long uptimes (like more then a year), the uptime command shows an exclamation mark. However, I cannot find a threshold or even the code

I sometimes noticed for long uptimes (like more then a year), the uptime command shows an exclamation mark. However, I cannot find a threshold or even the code for the exclamation mark at http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/uptime.c;h=c0869dbc4f939e65ea967b8ba840010e9009e40b;hb=HEAD http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f... -- does anybody have an idea about that? Is the exclamation mark intended to warn the admin to reboot (since there might be an updated kernel, etc.), or does the system wow itself? ;-)