Hi, The time package is too slow. :( A prime example is diffUTCTime, which invokes utcTimeToPOSIXSeconds, which in turn calls realToFrac :: DiffTime -> NominalDiffTime. The former of these two types notionally accounts for leap seconds, while the latter does not. Both are newtypes around Data.Fixed.Pico. Since realToFrac is used in the library only when we know the difference is within one day, the two notions coincide, and there is no need to account for leap seconds. Indeed it currently (or rather, its implementation of "fromRational . toRational") behaves as such. I propose we include some rewrite RULES to eliminate the round-trip via Rational. For the attached benchmark, I get a ~4x improvement in speed. Discussion period: 2 weeks. Cheers, /Liyang