NTP can adjust clock by minutes. User can adjust clock by minutes, days, months, or even years.

EDIT: Simple time torture script is added:

  for((I=0; I<100; I++)); do date --utc --set=@$(( `date --utc +%s`+1 )) ; date --utc --set=@$(( `date --utc +%s`-1 )) ; done