Before I get any further, let me tell you that some of what I’m going to say here was already covered by David Sparks in this post from almost six years ago. This was just a year and a half after the “beta” introduction of Siri with the iPhone 4S, and David was pleased with what Siri could do. I like a lot of what Siri can do with dates, too, but there are still some frustrating blind spots and inconsistencies. In fact, with one of David’s examples, Siri isn’t as convenient as it was six years ago.
Context has always been one of Siri’s weaknesses, and that’s where it failed Casey. Any normal human being would understand immediately that a question asked in January about days since a day in December is talking about the December of the previous year. But Siri ignores (or doesn’t understand) the word “since” and calculates the days until the next December 18.
Solid collection of examples of date calculations with Siri by Dr. Drang. As he notes, it's not that Siri can't answer complex questions involving dates – it's that you often have to phrase your questions with an exact syntax that a computer program can understand. This is frustrating because Apple promotes Siri as a smart assistant that can infer context without a refined syntax. I still run into a similar problem with time zone conversions; of course, the old trick I used to rely on no longer works for me unless I preface the question with "Ask Wolfram".