First, Thanks for your quick reply.
Yes, I tried it with setting "", but there is another obstacle here. If your custom field is nullable date, and you use the "format" option of your custom column, you'll get another null no matter how you change the data :). In my oppinion the best is to have a template which will make the whole gantt even more powerfull. I found such request in the Progress project map, but it is low voted and it's normal because of the low gantt usage after all.
Best Regards, Boyan.