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I am trying to do ephemeris calculations in R, which involves calculating an elapsed time. As illustrated below, difftime seems to have problems with my method, since the fractional day is sometimes the correct 0.5 and sometimes the incorrect 0.46. I am doing this on with R-2.6.1 on a powerpc-apple-darwin8.10.1 system. I get the same results for as.POSIXlt() instead of as.POSIXct(), but the