Richard Snodgrass of the University of Arizona championed BiTemporal technology in the 1990s. The proposed extensions to SQL:1999 to make implementing BiTemporal more graceful never made it through the standards-approval process. However, the main BiTemporal design principles have become even more applicable as business intelligence, OLAP, data warehouse/data marts, and data mining applications have proliferated. Snodgrass authored a book, Developing Time-Oriented Database Applications in SQL. The book is now out of print, but it developed something of a cult following: Used copies have sold for more than the original list price. Fortunately, Snodgrass has now posted it online free at: http://www.cs.arizona.edu/people/rts.