diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
+## [0.1.5.0] - 2022-11-15
+
+### Added
+
+ - `DayOfWeek`
+
 ## [0.1.4.0] - 2022-03-15
 
 ### Updated
@@ -32,6 +38,8 @@
 
 - Initial release, split out of hercules-ci-api
 
+[0.1.5.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.5.0
+[0.1.4.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.4.0
 [0.1.3.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.3.0
 [0.1.2.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.2.0
 [0.1.1.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.1.0
diff --git a/hercules-ci-api-core.cabal b/hercules-ci-api-core.cabal
--- a/hercules-ci-api-core.cabal
+++ b/hercules-ci-api-core.cabal
@@ -1,7 +1,7 @@
 cabal-version: 1.12
 
 name:           hercules-ci-api-core
-version:        0.1.4.0
+version:        0.1.5.0
 synopsis:       Types and convenience modules use across Hercules CI API packages
 category:       API, CI, Testing, DevOps, Nix
 homepage:       https://github.com/hercules-ci/hercules-ci-agent#readme
@@ -20,6 +20,7 @@
 
 library
   exposed-modules:
+      Hercules.API.DayOfWeek
       Hercules.API.Id
       Hercules.API.Name
       Hercules.API.Prelude
diff --git a/src/Hercules/API/DayOfWeek.hs b/src/Hercules/API/DayOfWeek.hs
new file mode 100644
--- /dev/null
+++ b/src/Hercules/API/DayOfWeek.hs
@@ -0,0 +1,80 @@
+{-# LANGUAGE DeriveAnyClass #-}
+
+-- This module pins down the details of week days in the API. It's boilerplate-y
+-- but that makes it robust.
+
+-- | A day of week type. Integer representations are not used in the API, but
+-- are provided for convenience and will not change.
+module Hercules.API.DayOfWeek
+  ( DayOfWeek (..),
+
+    -- * Conversions for @time@ package
+    toTime,
+    fromTime,
+
+    -- * Optional, stable, opinionated integer conversions
+    toNum,
+    fromNumMaybe,
+    fromNum,
+  )
+where
+
+import Control.DeepSeq (NFData)
+import Data.Aeson (FromJSON, ToJSON)
+import Data.Maybe (fromJust)
+import Data.Swagger (ToSchema)
+import qualified Data.Time
+import GHC.Generics (Generic)
+import Prelude (Eq, Integral, Maybe (..), Num (..), Show, mod)
+
+-- | Day of week representation used in the API.
+data DayOfWeek = Mon | Tue | Wed | Thu | Fri | Sat | Sun
+  deriving (Generic, Show, Eq, NFData, ToJSON, FromJSON, ToSchema)
+
+-- | Conversion to @time@ package representation.
+toTime :: DayOfWeek -> Data.Time.DayOfWeek
+toTime Mon = Data.Time.Monday
+toTime Tue = Data.Time.Tuesday
+toTime Wed = Data.Time.Wednesday
+toTime Thu = Data.Time.Thursday
+toTime Fri = Data.Time.Friday
+toTime Sat = Data.Time.Saturday
+toTime Sun = Data.Time.Sunday
+
+-- | Conversion from @time@ package representation.
+fromTime :: Data.Time.DayOfWeek -> DayOfWeek
+fromTime Data.Time.Monday = Mon
+fromTime Data.Time.Tuesday = Tue
+fromTime Data.Time.Wednesday = Wed
+fromTime Data.Time.Thursday = Thu
+fromTime Data.Time.Friday = Fri
+fromTime Data.Time.Saturday = Sat
+fromTime Data.Time.Sunday = Sun
+
+-- | Conversion to integers where 'Mon' -> 1, 'Sun' -> 7
+toNum :: Num n => DayOfWeek -> n
+toNum Mon = 1
+toNum Tue = 2
+toNum Wed = 3
+toNum Thu = 4
+toNum Fri = 5
+toNum Sat = 6
+toNum Sun = 7
+
+-- | Conversion from integers where @1 -> Just 'Mon'@, @7 -> Just 'Sun'@, @outside of 1..7 -> Nothing@
+fromNumMaybe :: (Num n, Eq n) => n -> Maybe DayOfWeek
+fromNumMaybe 1 = Just Mon
+fromNumMaybe 2 = Just Tue
+fromNumMaybe 3 = Just Wed
+fromNumMaybe 4 = Just Thu
+fromNumMaybe 5 = Just Fri
+fromNumMaybe 6 = Just Sat
+fromNumMaybe 7 = Just Sun
+fromNumMaybe _ = Nothing
+
+-- | Conversion from integers where @{...,-7,0,7,...} -> Just 'Sun'@, @{...,-6,1,8,...} -> Just 'Mon'@, etc.
+--
+-- Requires a sensible implementation of the 'Integral' 'mod' method that returns non-negative numbers.
+-- 'Integer', 'Int', 'Int8', etc are ok. So is the 'Word' family of types, though beware of 'minBound' overflows.
+fromNum :: (Integral n, Eq n) => n -> DayOfWeek
+fromNum n = fromJust (fromNumMaybe (1 + (n - 1) `mod` 7))