Change negative days by 1 to account for missing year zero

test/Database/PostgreSQL/Opium/FromFieldSpec.hs

@@ -249,6 +249,11 @@ spec = do
       -- Example from postgres doc page
       shouldFetch conn "SELECT date 'J2451187' AS day" [ADay $ fromGregorian 1999 1 8]
 
+      -- BC
+      -- See https://www.postgresql.org/docs/current/datetime-input-rules.html:
+      -- "If BC has been specified, negate the year and add one for internal storage. (There is no year zero in the Gregorian calendar, so numerically 1 BC becomes year zero.)"
+      shouldFetch conn "SELECT date 'January 1, 4710 BC' AS day" [ADay $ fromGregorian (-4710) 1 1]
+
   describe "FromField DiffTime" $ do
     it "Decodes the time" $ \conn -> do
       shouldFetch conn "SELECT time '00:00:00' AS difftime" [ADiffTime 0]
@@ -272,10 +277,8 @@ spec = do
       let ts2 = UTCTime (fromGregorian 1 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0)
       shouldFetch conn "SELECT timestamp '0001-01-01 00:00:00' AS utctime" [AUTCTime ts2]
 
-      {- TODO: How does "BC" work here?
-      let ts3 = UTCTime (fromGregorian 0 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0)
-      shouldFetch conn "SELECT timestamp 'January 1, 1 BC 00:00:00' AS utctime" [AUTCTime ts3]
-      -}
+      let ts3 = UTCTime (fromGregorian (-4710) 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0)
+      shouldFetch conn "SELECT timestamp 'January 1, 4710 BC 00:00:00' AS utctime" [AUTCTime ts3]
 
   describe "FromField RawField" $ do
     it "Simply returns the bytestring without decoding it" $ \conn -> do