DateAndTimeArithmetic

It is not within the scope of this library to handle the complex task of date and time arithmetic. It is recommended to use the 'Data.Time' library for handling dates and using 'Time' quantities only when time differences are involved in calculations with other quantities. In order to convert between the 'DiffTime' data type in the 'Data.Time' library and 'Time' quantities we provide the functions 'fromDiffTime' and 'toDiffTime'.

> fromDiffTime :: (Fractional a) => DiffTime -> Time a
> toDiffTime :: (Real a, Fractional a) => Time a -> DiffTime

TODO: Add example usage, e.g. "Greenwich hour angle".