I believe that R incorrectly formats POSIXct types with fractional seconds. I submitted this via R-bugs as an enhancement request and got brushed off with "we think the current behavior is correct -- bug deleted." While I am very appreciative of the work they have done and continue to do, I wanted to get other peoples' take on this particular issue, and perhaps advice on how to make the point more effectively.
Here is an example:
> tt <- as.POSIXct('2011-10-11 07:49:36.3')
> strftime(tt,'%Y-%m-%d %H:%M:%OS1')
[1] "2011-10-11 07:49:36.2"
That is, tt is created as a POSIXct time with fractional part .3 seconds. When it is printed with one decimal digit, the value shown is .2. I work a lot with timestamps of millisecond precision and it causes me a lot of headaches that times are often printed one notch lower than the actual value.
Here is what is happening: POSIXct is a floating-point number of seconds since the epoch. All integer values are handled precisely, but in base-2 floating point, the closest value to .3 is very slightly smaller than .3. The stated behavior of strftime() for format %OSn is to round down to the requested number of decimal digits, so the displayed result is .2. For other fractional parts the floating point value is slightly above the value entered and the display gives the expected result:
> tt <- as.POSIXct('2011-10-11 07:49:36.4')
> strftime(tt,'%Y-%m-%d %H:%M:%OS1')
[1] "2011-10-11 07:49:36.4"
The developers' argument is that for time types we should always round down to the requested precision. For example, if the time is 11:59:59.8 then printing it with format %H:%M should give "11:59" not "12:00", and %H:%M:%S should give "11:59:59" not "12:00:00". I agree with this for integer numbers of seconds and for format flag %S, but I think the behavior should be different