Calc/Performance/Repeated Querying of the Document Null Date
Date values are stored internally as the number of days since the null date. The null date can be configured per document. In the file format, date values are stored as XML Schema dates (basically ISO 8601). To convert between the two representations, the class SvXMLUnitConverter is used, and it needs the null date setting to do so. Currently the null date is queried from the document for each date value that is written, but because the setting applies to the whole document, it needs to be queried only once. This avoids a number of UNO calls when saving files with many date cells.