Normal household timers are not built to take the high inductive loads produced by HID ballasts. If used with a HID ballast, the contacts inside the timer that open and close the timed circuit will overheat and weld together, breaking the timer and leaving your grow lights permanently on.
Lumii 24 Hour Heavy Duty Timer Suitable for timing on 600w HID ballasts or up to 6 200w CFLs.