diff --git a/core-data.cabal b/core-data.cabal
--- a/core-data.cabal
+++ b/core-data.cabal
@@ -5,7 +5,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           core-data
-version:        0.3.4.0
+version:        0.3.6.0
 synopsis:       Convenience wrappers around common data structures and encodings
 description:    Wrappers around common data structures and encodings.
                 .
@@ -50,7 +50,7 @@
     , base >=4.11 && <5
     , bytestring
     , containers
-    , core-text >=0.3.7
+    , core-text >=0.3.8
     , hashable >=1.2
     , hourglass
     , prettyprinter >=1.6.2
diff --git a/lib/Core/Data/Clock.hs b/lib/Core/Data/Clock.hs
--- a/lib/Core/Data/Clock.hs
+++ b/lib/Core/Data/Clock.hs
@@ -46,8 +46,8 @@
 ) where
 
 import Control.Applicative ((<|>))
-import Core.Text.Rope
 import Core.Data.Format
+import Core.Text.Rope
 import Data.Aeson qualified as Aeson (FromJSON (..), ToJSON (..), Value (..))
 import Data.Aeson.Encoding qualified as Aeson (string)
 import Data.Aeson.Types qualified as Aeson (typeMismatch)
@@ -65,7 +65,8 @@
  )
 import Data.Int (Int64)
 import Data.Maybe (maybeToList)
-import Data.Time.Clock (UTCTime)
+import Data.Time.Calendar (Day)
+import Data.Time.Clock (UTCTime (UTCTime, utctDay, utctDayTime))
 import Data.Time.Clock.POSIX (
     POSIXTime,
     posixSecondsToUTCTime,
@@ -166,12 +167,18 @@
 nanosecond (who the hell has picoseconds of anything anyway?) if writing an
 instance of this type.
 
+The most important instance is probably the 'UTCTime' one, as many other
+Haskell libraries use this type to represent time.
+
 @since 0.3.3
 -}
 class Instant a where
     fromTime :: Time -> a
     intoTime :: a -> Time
 
+{- |
+Number of nanoseconds since the epoch.
+-}
 instance Instant Int64 where
     fromTime = unTime
     intoTime = Time
@@ -193,6 +200,11 @@
 convertToPosix :: Time -> POSIXTime
 convertToPosix = fromRational . (/ 1e9) . fromIntegral . unTime
 
+{- |
+Convert to the elapsed time with sub-second precision type from __hourglass__,
+giving you ready access to that library's time formatting and calendar date
+manipulation functions.
+-}
 instance Instant H.ElapsedP where
     fromTime = convertToElapsed
     intoTime = convertFromElapsed
@@ -207,6 +219,18 @@
 convertToElapsed (Time ticks) =
     let (s, ns) = divMod ticks 1000000000
      in H.ElapsedP (H.Elapsed (H.Seconds (s))) (H.NanoSeconds (ns))
+
+{- |
+This instance may be useful if you need to work with calendar dates with
+functions from __time__. From here you would probably be interested in
+'Data.Time.Calendar.toGregorian'. If you convert from a 'Day' it will be the
+timestamp of midnight 00:00:00.0 on that date.
+
+@since 0.3.5
+-}
+instance Instant Day where
+    fromTime = utctDay . fromTime
+    intoTime x = intoTime (UTCTime{utctDay = x, utctDayTime = 0})
 
 instance Aeson.ToJSON Time where
     toEncoding = Aeson.string . H.timePrint ISO8601_Precise . convertToElapsed
diff --git a/lib/Core/Encoding/External.hs b/lib/Core/Encoding/External.hs
--- a/lib/Core/Encoding/External.hs
+++ b/lib/Core/Encoding/External.hs
@@ -54,6 +54,8 @@
 import Data.ByteString.Builder qualified as Builder
 import Data.Int (Int32, Int64)
 import Data.Scientific (FPFormat (Exponent), Scientific, formatScientific)
+import Data.Time.Calendar qualified as Base (Day)
+import Data.Time.Format.ISO8601 qualified as Base (formatParseM, formatShow, iso8601Format)
 import Data.UUID qualified as Uuid (UUID, fromText, toText)
 import Text.Read (readMaybe)
 
@@ -200,6 +202,17 @@
 instance Externalize Time where
     formatExternal = intoRope . show
     parseExternal = readMaybe . fromRope
+
+{- |
+Days are formatted as per ISO 8601:
+
+@
+2022-06-20
+@
+-}
+instance Externalize Base.Day where
+    formatExternal = intoRope . Base.formatShow Base.iso8601Format
+    parseExternal = Base.formatParseM Base.iso8601Format . fromRope
 
 {- |
 Numbers are converted to scientific notation: