Ok, I think I see the problem. To me, MSb (Most Significant bit) isn’t an ordering at all, just a label that one particular bit has. To specify an ordering, you’d also need to say whether that bit comes first or last. This concept doesn’t exist in computer memory because, as previously mentioned, bits in a byte aren’t ordered in memory. I was thinking of the individual digits in a field (each Y in YYYY) as separate bytes in a word, so endianness order makes sense to think about; separate fields in this analogy were contiguous like struct fields. I think my mental model is sensible, since ISO 8601 is fundamentally a sequence of characters, which are all in an absolute order.