Apart from the calendar issue, the more fundamental question (for me) is whether this function should be making changes to the cube in-place or not?

If it is in-place then we need to advertise this in the doc-string and **not** return anything; otherwise it's confusing.

The alternative is to return a copy of each cube with the time units aligned... however, this might not be wise given that the payload of a cube may be loaded. So I'm voting for in-place and not returning anything.