I'm an instructor, and I set my assignment due times to coincide with class meeting times so I can ensure students have done prep work before class time. My classes don't always meet at the same time each semester and I haven't found a way to change Assignment due times or to-do times for Pages when importing content. I tried setting the class meeting time as part of the course start date when importing, but this didn't work; all the content imported with the same due time as the course that was imported.
For Assignments, the fastest (but still tedious) way I've found to change the times is to click on each individual assignment using Calendar and edit it, and for Pages is to edit all the Pages on the list. This takes forever, as my courses have about a dozen short assignments each week. Am I missing something? Haven't found anything in the Guides or user posts.
There is a too for bulk editing assignment due dates and times, so that you don't have to do it through the calendar. See https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-bulk-update-due-dates-and-availability-... . You will still have to manually type in the new time, but it's quicker that scrolling through the calendar and opening each calendar entry individually.
I don't really see an option for changing dates and times on pages in bulk, though.