For certain BTs (e.g., BT-131 and BT-1311), the time fields are forbidden as long as the date fields are not filled in. In my opinion, this should be the case for every date+time combination, otherwise it is possible to specify only a time without a date.
There is currently no such rule for at least the following BTs:
BT-800-Lot
BT-13-Lot
BT-13-Part
BT-132-Lot
BT-630-Lot (only on certain subtypes)