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Provide ability to get exact results when use timestamp arithmetic with fractional part of second (DATEADD(), DATEDIFF()).

SQL> select dateadd(-55.499 second to timestamp '01.01.2000 00:00:55.499') from rdb$database; 2000-01-01 00:00:00.4990 SQL> select dateadd(-55.501 second to timestamp ...
GitHub

-search-sql-dateadd.md

The DATEADD function performs time and date calculations for matching properties having date types. Use the DATEADD function to obtain dates and times in a specified amount of time before the present.