[R] Wrong computation of time differenze in POSIXct - additional digits

[Julia]

Hello, 

I wanted to compute the time differenze between to times:

first =as.POSIXct( "2012-06-15 16:32:39.0025 CEST")
second  = as.POSIXct("2012-06-15 16:32:39.0086 CEST")
second - first

The result is
Time difference of 0.006099939 secs

instead of just 0.0061 secs
So R adds aditional  numbers after the result. 
I know I could round it in this case.
But I am working with a large data set and need to always get the 
correct result. 

difftime() does not work correct either. 

Has anybody a suggestion how to get the correct result?

Thank you
Julia