Custom time field won’t sort regardless of format

I don’t see any reason why your code should not work, and when I test things it does work. I think that one of the following is happening:

  1. Your meta_key is wrong. Perhaps it is prepended by a _ for
  2. You don’t have correct, or consistent, timestamps
  3. You have sticky posts getting in the way
  4. There is a filter acting on the query.

To solve #3, add 'ignore_sticky_posts' => true, to the arguments. For the others, I can’t say. I don’t the ability to investigate those possibilities.