I find Tumblr’s schedule functionality really frustrating. You can schedule posts by using a relative date e.g. Next Tuesday, 10am (which is already inserted), or if you want to schedule a post for much further into the future, you can write 3 weeks Monday, 10am. I don’t think that this is a very intuitive method.
What is hard to tell from the above UI is that you can also schedule posts using the date format of MM/DD/YYYY Time. I only found this out by searching their help section.
I’d be interested to know what their user testing shows about this design pattern and whether the date format is the one that people are expecting to see.