DiffTime is:exact

data DiffTime
time Data.Time.Clock, distribution-opensuse OpenSuse.Prelude
This is a length of time, as measured by a clock. Conversion functions such as fromInteger and realToFrac will treat it as seconds. For example, (0.010 :: DiffTime) corresponds to 10 milliseconds. It has a precision of one picosecond (= 10^-12 s). Enumeration functions will treat it as picoseconds.
data DiffTime
rio RIO.Time
This is a length of time, as measured by a clock. Conversion functions will treat it as seconds. It has a precision of 10^-12 s.
data () => DiffTime
time-compat Data.Time.Clock.Compat, rebase Rebase.Prelude
This is a length of time, as measured by a clock. Conversion functions such as fromInteger and realToFrac will treat it as seconds. For example, (0.010 :: DiffTime) corresponds to 10 milliseconds. It has a precision of one picosecond (= 10^-12 s). Enumeration functions will treat it as picoseconds.
data DiffTime
thyme Data.Thyme.Clock
An interval or duration of time, as would be measured by a stopwatch. DiffTime is an instance of AdditiveGroup as well as VectorSpace, with Rational as its Scalar. We do not provide Num, Real, Fractional nor RealFrac instances here. See Data.Thyme.Docs#spaces for details. 
> fromSeconds' 100 :: DiffTime
100s
> fromSeconds' 100 ^+^ fromSeconds' 100 ^* 4
500s
> fromSeconds' 100 ^-^ fromSeconds' 100 ^/ 4
75s