First wave of 9.1 R3 was rolled out Tuesday. In this release we changed most of our archive columns from type DateTime to Date because this is how they should be displayed and handeled.
For partner apps who parse DateTime values without using our parsers, the change from prefix [DT: to [D: may cause issues.
Do note - the field will still return the full datetime value, it's just the prefix that has changed so it now returns info on how the field should be displayed and handled. A blogpost by Dr. Marek Vokáč with more info will come soon.