date --set='-2 years' <===> to set the clock back two years, leaving all other elements identical. You can change month and day of month the same way.
date -s "2014-12-25 $(date +%H:%M:%S)" <===> will change the date, but keep the time. the individual components are %Y, %m, %d, %H, %M, and %S.
hwclock --show <===> print the hardware time
hwclock --hctosys <===> set the hardware clock to the current system time
hwclock --systohc <===> set system time to the hardware clock
The date formats are:
%Y - Year
%m - Month
%d - Day
%H - Hour
%M - Minute
