Change negative days by 1 to account for missing year zero

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -25,6 +25,7 @@ import Data.Time
   , fromGregorian
   , picosecondsToDiffTime
   , timeToTimeOfDay
+  , toGregorian
   )
 import Data.Text (Text)
 import Data.Word (Word32, Word64)
@@ -129,13 +130,19 @@ instance FromField Bool where
 postgresEpoch :: Day
 postgresEpoch = fromGregorian 2000 1 1
 
+fromPostgresJulian :: Integer -> Day
+fromPostgresJulian x
+  | year <= 0 = fromGregorian (year - 1) month dayOfMonth
+  | otherwise = day
+  where
+    day = addDays (fromIntegral x) postgresEpoch
+    (year, month, dayOfMonth) = toGregorian day
+
 -- | 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 <$> intParser @Int32
-    where
-      fromJulianDay x = addDays (fromIntegral x) postgresEpoch
+  parseField = fromPostgresJulian . fromIntegral <$> intParser @Int32
 
 -- | See https://www.postgresql.org/docs/current/datatype-datetime.html.
 -- Binary format: https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/adt/date.c;h=ae0f24de2c3c54eb6d0405cdb212597c2407238e;hb=HEAD#l1542.
@@ -162,7 +169,7 @@ instance FromField UTCTime where
       fromPostgresTimestamp ts =
         let
           (days, microseconds) = ts `divMod` (86400 * 1000000)
-          day = addDays (fromIntegral days) postgresEpoch
+          day = fromPostgresJulian $ fromIntegral days
           time = picosecondsToDiffTime $ fromIntegral microseconds * 1000000
         in
           UTCTime day time