One date issue and one date suggestion.
Issue: ToDoIst populates incorrect date.
I attempted to add a task. I clicked on the date widget, scrolled to January 2013, and attempted to select "January 31". ToDoIst populated the date "Jan 1 2013" (i.e., it should have populated "Jan 31 2013").
Suggestion: Allow commas.
In troubleshooting the above, I found that ToDoIst does not accept dates in the format of "January 31, 2013." It seems the comma isn't accepted. I'm sure you've considered this, but I'd recommend allowing for this, since it is typical date syntax.