i take it its being used in the states with a voltage rating of 125v?
i got a timer from a grow shop for £5 ($10-12)to run with my 250 watt MH and it works a treat
some of the more expensive shit may last a bit longer but will cause much confusion and could blow your ballast as soon as you switch it on,did you buy the timer from a grow shop?if so phone them up and ask about it,they are there to help