The digital/analogue ones sold around are not designed to run metal halide lights, as when the lights first turn on the inrush current can be huge causing the switch contacts in the timers to stick together, or to burn apart.
I have tried the cheap ones, as well as the ones designed for spa pool motors etc. I have even tried solid state relays rated at 40 amps and they also fail.
This was happening on my 150 watt as well as my 250 watt ballasts.
As puttputt has said, the only other way I can see it working is to wire a normal relay with a timer, and make sure the contacts are rated for a high current.
Just did a web search and found ...
3500 watt timer, sold by store that sells equipment for hydroponics etc.
https://secure.sog.co.nz/store.htm Goto electrical and scroll down to timers.