This tip comes from Eric Nordmoe, and could save you some time if you regularly import your previous course materials into your new Moodle course page.
View the full walk-through on how to change all assignment/activity dates at once, or read the brief summary below.
You can always change assignment dates by editing them individually. However, you can also change multiple dates (from multiple assignments) simultaneously from the "Dates" screen.
To access the "Dates" screen, visit your Moodle course page and go to ADMINISTRATION > Reports > Dates.
Thanks for the tip, Eric!