I agree with everything you said and don't think this format is bad at all, but regarding your "Y10K problem" note, I actually prefer PHP date format, that is "Y/y" instead of "yyyy/yy".
First of, I'm sure you need year far more often than "week year", so the less intuitive encoding for the latter is acceptable and I have to admit that confusing letter case is pretty easy when the result is identical for most of the time (so simple "run it, see if it works" won't help, and that's exactly how everybody does programming).
Second, I don't imagine I'll ever want to display last three symbols of the year instead of the full year, and I most of the time don't want to get "0476" instead of "476" — and if I do, I'm actually OK with post-processing for these special occasions. The only thing I actually want is full year format (whatever long it is) or, rarely, short (2-letter) year format (which I actually think is stupid and never use it except somebody specifically asks me to). So while I'm absolutely clear with what "Y/y" does, I have no idea what 'yyy' would do for 4-digit long year or what 'yyyy' would do for 3 or 5 digit long year for that matter.
First of, I'm sure you need year far more often than "week year", so the less intuitive encoding for the latter is acceptable and I have to admit that confusing letter case is pretty easy when the result is identical for most of the time (so simple "run it, see if it works" won't help, and that's exactly how everybody does programming).
Second, I don't imagine I'll ever want to display last three symbols of the year instead of the full year, and I most of the time don't want to get "0476" instead of "476" — and if I do, I'm actually OK with post-processing for these special occasions. The only thing I actually want is full year format (whatever long it is) or, rarely, short (2-letter) year format (which I actually think is stupid and never use it except somebody specifically asks me to). So while I'm absolutely clear with what "Y/y" does, I have no idea what 'yyy' would do for 4-digit long year or what 'yyyy' would do for 3 or 5 digit long year for that matter.