Unfortunately this game needs a guide that it barely has. Some missions you can only do if you have done other missions before. (for example, to attend yoga classes with Tris you must have 20 in the yoga stat otherwise you will not be able to advance)
You must go in the afternoon but if you change the time of day you must do so from outside the hospital so that the game recognizes it. (if you see that you still can't, try that same thing at night but I seem to remember that it is in the afternoon)