According to the BACnet protocol documentation:

The default value to be used for the presentValue property when all command priority values in the priorityArray property have a NULL value.

Property relinquishDefault of object datetimeValue can be read and written using a single IOTag.

Text Parameters

Numeric Parameters

Tag Value

A Date, Null or String (Xml) value representing a BACnet date/time.

•A null value means the date/time is empty/not set;

•A date/time value representing a precise date and time;

•A character string containing an XML with a value of type <BACnetDateTime>.