How cold does it have to be before diesel gels?

Thanks for all the responses. Always learn alot from these posts.
It's a bit warmer here now so I'm not so worried, but I need to be prepared.
The first part of my question was about running the generator in cold weather, the second part was more general running the engine in cold weather.
Just to be safe I'll fill the tank as soon as I can and dump a little treatment in to be safe.
Thanks All