Avoid using a «Most people won't need this» module and use an intermediate value that is not meant to represent a duration

src/Article.hs

@@ -10,11 +10,13 @@ module Article (
 import Control.Applicative ((<|>))
 import Data.Map (Map)
 import qualified Data.Map as Map (fromList, alter)
-import Data.Time (defaultTimeLocale, getCurrentTimeZone, parseTimeM, timeZoneOffsetString)
-import Data.Time.Clock.POSIX (POSIXTime, utcTimeToPOSIXSeconds)
-import Foreign.C.Types (CTime)
+import Data.Time (
+      UTCTime, defaultTimeLocale, getCurrentTimeZone, parseTimeM
+    , timeZoneOffsetString
+  )
+import Data.Time.Clock.System (systemSeconds, utcToSystemTime)
+import System.Directory (getModificationTime)
 import System.FilePath (dropExtension, takeFileName)
-import System.Posix.Files (getFileStatus, modificationTime)
 import Text.ParserCombinators.Parsec (
       ParseError
     , Parser
@@ -71,16 +73,17 @@ eol = try (string "\r\n") <|> string "\r" <|> string "\n" <?> "newline"
 no :: String -> Parser String
 no = many1 . noneOf
 
-setDate :: String -> CTime -> Metadata -> Metadata
+getTimestamp :: UTCTime -> String
+getTimestamp = show . systemSeconds . utcToSystemTime
+
+setDate :: String -> String -> Metadata -> Metadata
 setDate tzOffset defaultDate = Map.alter timeStamp "date"
   where
     formats = ("%Y-%m-%d" ++) . (++ " %z") <$> ["", " %H:%M"]
-    epoch = show . (truncate :: POSIXTime -> Integer) . utcTimeToPOSIXSeconds
-    timeStamp Nothing = Just $ show defaultDate
-    timeStamp (Just date) =
+    timeStamp = maybe (Just defaultDate) $ \date ->
       let dates = [date, date ++ " " ++ tzOffset] in
       let parsedTimes = parseTimeM True defaultTimeLocale <$> formats <*> dates in
-      foldr (<|>) (timeStamp Nothing) (fmap epoch <$> parsedTimes)
+      foldr (<|>) (timeStamp Nothing) (fmap getTimestamp <$> parsedTimes)
 
 makeArticle :: FilePath -> (Metadata -> Metadata) -> ProtoArticle -> (String, Article)
 makeArticle filePath metaFilter (title, metadata, bodyOffset, body) = (
@@ -97,7 +100,7 @@ makeArticle filePath metaFilter (title, metadata, bodyOffset, body) = (
 at :: FilePath -> IO (Either ParseError (String, Article))
 at filePath = do
   tzOffset <- timeZoneOffsetString <$> getCurrentTimeZone
-  fileDate <- modificationTime <$> getFileStatus filePath
+  fileDate <- getTimestamp <$> getModificationTime filePath
   let build = makeArticle filePath (setDate tzOffset fileDate)
   fmap build . parse articleP filePath <$> readFile filePath