Redmine

I already asked this question on stackoverflow, but that might have not been the ideal place for this specific question.
So I hope somebody in here can be of help with my problem:

In my current project set up I have a version with a set due date. Now all the tasks for that version have an assigned start/end date (which in this specific case is the same day). E.g.

task x     -> start 20.3. / due 20.3.
task y     -> start 21.3. / due 21.3. 
task [...] -> start [...] / due [...]

Now it could happen that I am not able to finish a certain task as planned on the given day.

The only option I found to postpone a set of tasks was to bulk edit the start/end date, which only gives me the option to set the same start/end date for all selected tasks. Is there a way to simply postpone a set of tasks by amount x?

E.g.:

task #1 -> due someday+x
...     -> ....
task #N -> due otherday+x 

What would be the best approach for the problem described above? How is everybody else postponing tasks? Is there a "canonical" way to do it? Am I even doing it wrong? Maybe there is a plugin that solves this specific problem?

Help is highly appreciated. Thx in advance