Postponing several tasks on Gantt in one shot

When autoplanning is not used (manual planning, so "left work"=0 or no resources assigned to tasks), it would be very useful to have the possibility to manually postpone several activities all together.
Otherwise, if you have a big Gantt and the overall project is postponed it's a nightmare to manually shift all the activities.
Drag and drop would be the best, but as a workaround you could set that the user can use the already present feature to modify several tasks together, and symply write "+ number of days" in the dates that have to be modified.

Like Start date = "+5".

So that all the selected tasks get updated relatively to their initial start date.

Thanks

Alessio