Copying assignments - start/end date&time

Hi,

Is there a way to change the start/end date and time of a copied assignment ?

I copied the assignments of an old classroom to a new one and I can only change the limit date&time.

+ is there a way to change the date of all the assignments without changing the time ? This would make much easier to change the date of the entire list of assignments with one-click action without loosing their order (which can only be preserved by keeping different time for them).

Thank you (and please excuse my poor english).