Hi @Anon48723,
I see what you mean. In "assign to" areas, even though the dropdown menus only show 30 minute increments, you can manually type any other time you want (say, 11:50 pm). For ungraded discussions, the to-do time box overrides manual things you type in. If you have access, the best thing might be to contact Canvas support and let them know about this issue. I have a feeling the intended behavior is to let you type in any time you want, and this is actually a bug they can address. The community here is comprised of mostly other Instructure product users, so filing a support ticket is much more likely to get this addressed than the community post. The good thing about the community post is that others like me may come across this thread, agree with us, and file their own support tickets as well, which will make it even more likely to get addressed quickly.
Hope this helps a bit, and I do hope Instructure addresses this quickly!
-Chris