Basically title. 2019 edition of the Standard denotes the "T" prefix to time as mandatory (except in "unambiguous contexts"):

01:29:59 is now actually T01:29:59, with the former form now designated as an alternative

But date does not have a "D" prefix, not even in "ambiguous contexts".

1973-09-11 never needs to be something like eg.: D1973-09-11

Anyone know the reasoning behind this change and what is the intended use? The only time-only format with separators that I can think would be undecidable in ambiguous contexts would be hh:mm which I guess could be mistaken for bible verses?