Chronos is a performance-oriented time library for Haskell, with a straightforward API. The main differences between this and the time library are:

• Chronos uses machine integers where possible. This means that time-related arithmetic should be faster, with the drawback that the types are incapable of representing times that are very far in the future or the past (because Chronos provides nanosecond, rather than picosecond, resolution). For most users, this is not a hindrance and the tradeoff is worthwhile.
• Chronos provides 'ToJSON'/'FromJSON' instances for serialisation.
• Chronos provides 'Unbox' instances for working with unboxed vectors.
• Chronos provides 'Prim' instances for working with byte arrays/primitive arrays.
• Chronos uses normal non-overloaded haskell functions for encoding and decoding time. It provides attoparsec parsers for both 'Text' and 'ByteString'. Additionally, Chronos provides functions for encoding time to 'Text' or 'ByteString'. The time library accomplishes these with the Data.Time.Format module, which uses UNIX-style datetime format strings. The approach taken by Chronos is faster and catches more mistakes at compile time, at the cost of being less expressive.

Jacob Stanley has written a blog post comparing the features and performance of time, thyme, and chronos. It has a good bulleted breakdown of why you may want to use each library along with some benchmarks.

Benchmarks of chronos against time and thyme.

Doctest used to be provided as a test suite, but doctest-0.20 and higher do not require this to be run. To run the doctests, make sure you have doctest on your path (i.e. run cabal install doctest), and then run:

cabal build
cabal repl --build-depends=QuickCheck --with-ghc=doctest --repl-options='-fno-warn-orphans'

This runs incredibly slowly, but it works for now. Doctest is not run by CI, so if you make a change that adds more doctests, it needs to be run by hand by someone. (The maintainer is happy to do this if you're on a platform where doctest is finicky.)