Chapter 41. Temporal Data
Clifford Simak wrote a science fiction novel entitled Time is the Simplest Thing in 1977. He was wrong. And the problems did not start with the Y2K problems that we had in 2000, either. The calendar is irregular and the only standard unit of time is the second; years, months, weeks, hours, minutes, and so forth are not part of the metric system but are mentioned in the ISO standards as conventions.
SQL-92 added temporal data to the language, acknowledging what was already in most SQL products by that time. The problem is that each vendor made a trade-off internally and the syntax was proprietary. DB2 follows the ANSI/ISO syntax, SQL Server is just catching up in their 2008 version, and Oracle is still proprietary.