Such a field would be great. Often  you get an issue
with a due date to solve it. 

But i cannot find it in the Roundup Issue Tracker.

Is there already a solution?

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i forgot.. to add it to the classic tracker... i found the
description how to do it, but i think its a usefull field

By default roundup comes with a minimal set of fields. You can customize the schema as described at:

http://roundup.sourceforge.net/doc-1.0/customizing.html

There is certainly room for an argument about what default fields roundup should come with but I think it is unlikely to change at this point without some significant discussion first.

If that day happens we can revisit what fields should be added. Until then I'm closing this (rather old) suggestion.