Implement date -> Day decoding

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -11,7 +11,9 @@ import Data.Attoparsec.ByteString (Parser)
 import Data.Bits (Bits (..))
 import Data.ByteString (ByteString)
 import Data.Functor (($>))
+import Data.Int (Int32)
 import Data.Proxy (Proxy (..))
+import Data.Time (Day (..))
 import Data.Text (Text)
 import Data.Word (Word32, Word64)
 import Database.PostgreSQL.LibPQ (Oid)
@@ -107,3 +109,11 @@ boolParser = AP.choice
 instance FromField Bool where
   validOid Proxy = Oid.boolean
   parseField = boolParser
+
+-- | See https://www.postgresql.org/docs/current/datatype-datetime.html.
+-- Relevant as well: https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/adt/datetime.c;h=267dfd37b2e8b9bc63797c69b9ca2e45e6bfde61;hb=HEAD#l267.
+instance FromField Day where
+  validOid Proxy = Oid.date
+  parseField = fromJulianDay . fromIntegral <$> intParser @Int32
+    where
+      fromJulianDay x = ModifiedJulianDay $ x + 51544

lib/Database/PostgreSQL/Opium/Oid.hs

@@ -48,3 +48,7 @@ doublePrecision = eq $ Oid 701
 -- | Boolean
 boolean :: Oid -> Bool
 boolean = eq $ Oid 16
+
+-- | Single days/dates.
+date :: Oid -> Bool
+date = eq $ Oid 1082