avoid overwriting timestamps if they are forced by markdown already

Types.hs

@@ -132,7 +132,8 @@ siteOptions' = do
   _timestampSuffix <-
     strOption $
     long "timestamp-prefix" <>
-    help "Timestamp file suffix for markdowns" <> value ".timestamp" <> showDefault
+    help "Timestamp file suffix for markdowns" <>
+    value ".timestamp" <> showDefault
   _dumpFinalState <-
     switch $
     long "dump-state" <>

cards/404.md

@@ -2,6 +2,8 @@
 mount: /404
 title: Page not found
 search: off
+toc: off
+timestamp: null
 ---
 
 # Page not found!

cards/index.md

@@ -3,6 +3,7 @@ mount: /
 title: Home
 template: index.html
 toc: off
+timestamp: null
 ---
 
 #### What are How-To Cards?

cards/search.md

@@ -4,6 +4,7 @@ title: Search
 template: search.html
 search: off
 toc: off
+timestamp: null
 ---
 
 # Search

site.hs

@@ -182,7 +182,9 @@ timestampFile fp = do
 
 -- | If a timestamp file for the page exists, add the timestamp metadata.
 addTimeMeta :: PageInfo -> Y.Value -> Site Y.Value
-addTimeMeta pi m'@(Y.Object m) = do
+addTimeMeta pi m'@(Y.Object m)
+  | "timestamp" `KM.member` m = pure m' -- do not overwrite the timestamp if present
+  | otherwise = do
     tspath <- timestampFile $ pi ^. pagePath
     io $
       ifM