We just disagree, then.

To pick a tiny, arbitrary example from the keynote, he criticises this method[0]…

  def age(now = Date.today)
    now.year - birthday.year
  end

…as opposed to his preferred version…

  def age
    Date.today.year - birthday.year
  end

“Is [the method with the parameter] better? Is it simpler? Is it clearer?” He makes fun of it as though the second version is obviously simpler and clearer, presumably because it uses fewer characters/parameters/concepts, but in reality it’s not obvious. It depends what you want!

The first version makes it “clearer” that the method’s result is date-dependent, which makes it “simpler” to understand how it will behave as part of a larger system (e.g. is it cacheable?); this is a win for composability. If you want to call it from inside another method, you’ll need to get a date from somewhere — maybe you’ll already have the appropriate date to hand, or maybe you’ll choose to pass that responsibility onto your caller in turn, or maybe you’ll decide that this is the right place to reach for Date.today. Either way, you get a chance to think about it, and to be aware of how another part of the software is going to behave without needing to go and look at its source code. (This argument is essentially “referential transparency is good”.)

Now, it’s entirely plausible that you don’t care about that benefit, and you’d rather have a shorter method that works in the easiest possible way without regard for referential transparency, because your system is small, or this method is hardly called anywhere, or everything else in the application is time-dependent anyway so you don’t need to be reminded of it. That’s fine too! But DHH doesn’t go into any detail on the tradeoff; he just makes fun of the version that takes an argument, because it’s testable for the sake of it, and why would anyone bother with that?

He doesn’t seem interested in exploring the situation, or in interrogating what testability implies in this case, only in laying out his prejudices as if they’re indisputable common sense. They’re not.

[0] Since this is just example code, it’d be churlish to point out that it’s not the right way to calculate someone’s age in the first place.

First method works just fine if I want to calculate what was his age last year, and what will be his age at the year of 2021. The second method doesn't, so these are not even functionally equivalent.