Thought the settings for the course show that the term end date is October 2019, why can't I extend an assignment due date to August 2019?
There are two things I can think of off the top of my head that might cause this. I'll go into detail on one since I know what's doing. I'll mention the other one, but I don't have any experience with it.
Determining the dates a course is available can be tricky and people often think they've changed the dates when they haven't.
The default dates for a term are set at the account level. Only a Canvas admin can set these. They do this by going to Admin > account name > Terms.
For example, here is our Summer 2019 term dates: Students can access the course between June 2 and August 1 unless other settings overrule these values.
The term dates may be overridden by the Course settings. A teach can set these by going to their course Settings page.
If the dates are blank, as shown below, then the term dates are used.
What confuses people is when the dates are entered and Canvas still uses the term dates. In the screenshot below, I've entered July 1 and July 31, but Canvas still uses June 2 to August 1, which are the term settings.
The reason Canvas still uses the term settings is because whatever you enter is considered an override, but you have to check the box to say "Student can only participate ..." for the override to take effect.
When you check that box, as shown below, then the course is available for the dates you've specified.
There is a third level of dates, though, that might override that setting. It's not likely unless you were using a differentiated assignment and the entire section or the student you were setting was in such a section, was overridden by going to the course settings > sections page and changing the dates there.
If the dates are set for the section, they may override the course and term settings.
This screenshot shows the dates of June 1 to June 30, but the "Students can only participate ..." checkbox is not checked, so they are meaningless and ignored by Canvas. The students will get into the course based on the course settings or term settings.
If I check the student participation box, then the dates will take effect for the section.
Section dates override course dates and course dates override term dates. However, section and course overrides aren't recognized unless you check the appropriate student participation date.
What I would do is check to make sure that box is checked wherever you think you've set the dates.
The second thing I can think of is grading periods. I'm in higher ed and don't use these, but you might. With grading periods, there are restrictions about what happens with an assignment. Something about you can't extend the dates in the grading period or something like that. I really don't know much about this, so I'll just point you to the documentation in the Canvas Instructor Guide: How do I use grading periods in a course?
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