Segmental timers are accurate to 15 minutes. These timers are perfect for controlling lighting and hydroponic feeding systems. Segmental timers from Grasslin are very easy to use you just set the15 minute segments around the edge as on or off and away you go.
To safely use timers with high powered lighting you will need to also use a contactor or relay to protect the timer from the electrical surge of HPS lighting startup. Without a contactor relay the timer will get fried and your light will not turn off (or no).