That (some date + 1 month) + 1 month ≠ some date + 2 months has always been true for floating numbers. This is exactly why you never trust fast matrix mul

That (some date + 1 month) + 1 month ≠ some date + 2 months has always been true for floating numbers. This is exactly why you never trust fast matrix multiplications---they rely on cancelations of the form (a + b) - b = a + (b - b) = a. I would argue that 1 month is just having too few significant digits. You can even implement it as returning 30 with probability 60% and returning 31 with probability 40%. Then on average you would have 1 year ≈ sum([1 month] * 12)