/dev/rtc
/dev/nvram

The rtc file behaves just like /dev/time (see cons(3)). The real-time clock is maintained on-board; /dev/time is set from the file server. Neither is necessarily more accurate.

The nvram file provides (if permission allows) access to the local non-volatile RAM. For example, boot(8) reads the machine's key from there (see auth(8)).