yyyy-mm-dd hh:mm:ss.fffffffff

does anyone understand what does ‘f’ mean?? milli seconds ??

Represents a digit of a fraction of a second. I’m guessing you’re looking at this in the context of a Microsoft technology (.NET or SQL Server?). Here’s a blog post that explains a bit…

http://blogs.msdn.com/bclteam/archive/2005/03/09/391418.aspx