1. Create an Empty (null) by pressing Shift A and select Plain Axes.

2. Select all objects and then select Empty as your last selection. (If you use box select, deselect empty and reselect it again. Selecting the Empty last will make the empty the parent and the object the child.)

3. Press Ctrl P to Parent object to Empty and select Object (Keep Transformation).

4. Press N to bring up the transform panel. (If you can not see the transform information click on the Item tab on the side.

5. Go the Rotate Z field and right mouse click and select Insert Single Keyframe.

6. On the [...]