diff --git a/package.yaml b/package.yaml
index fbf8ed8..1f1d3de 100644
--- a/package.yaml
+++ b/package.yaml
@@ -23,6 +23,7 @@ dependencies:
 - base >= 4.7 && < 5
 - brick
 - ConfigFile
+- containers
 - megaparsec
 - optparse-applicative
 - sqlite-simple
@@ -33,7 +34,6 @@ dependencies:
 - vty
 - hspec
 - QuickCheck
-  #- containers
   #- unordered-containers
   #- transformers
   #- async
diff --git a/src/TaskMachine/Todotxt/Dates.hs b/src/TaskMachine/Todotxt/Dates.hs
new file mode 100644
index 0000000..e39d7bd
--- /dev/null
+++ b/src/TaskMachine/Todotxt/Dates.hs
@@ -0,0 +1,97 @@
+-- | Read, parse and write files in the <https://github.com/todotxt/todo.txt todo.txt> format.
+
+
+module TaskMachine.Todotxt.Dates
+  ( Dates()
+  , showDates
+  -- * Modification
+  , creationDate
+  , completionDate
+  , setCreationDate
+  , setCompletionDate
+  -- * Parsing
+  , day
+  , dates
+  ) where
+
+import           Control.Monad
+import           Data.Void
+
+import           Data.Time.Calendar
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+-- | The combination of creation and completion date of a task.
+--
+-- These two dates were combined because, according to the
+-- <https://github.com/todotxt/todo.txt/blob/master/README.md#todotxt-format-rules todo.txt format rules>,
+-- the creation date "must be specified if completion date is".
+data Dates
+  = NoDate
+  | CrDate Day
+  | CrCoDate Day Day
+  deriving (Show)
+
+{- Modification -}
+
+-- | Convert a 'Dates' to a string representation that can be used inside a todo.txt task
+-- and parsed by 'dates'
+showDates :: Dates -> String
+showDates NoDate                         = ""
+showDates (CrDate creation)              = show creation
+showDates (CrCoDate creation completion) = show creation ++ " " ++ show completion
+
+-- | Retrieve the creation date, if one exists
+creationDate :: Dates -> Maybe Day
+creationDate (CrCoDate day _) = Just day
+creationDate (CrDate day)     = Just day
+creationDate NoDate           = Nothing
+
+-- | Retrieve the completion date, if one exists
+completionDate :: Dates -> Maybe Day
+completionDate (CrCoDate _ day) = Just day
+completionDate _                = Nothing
+
+-- | Set the creation date to a specific value
+setCreationDate :: Day -> Dates -> Dates
+setCreationDate creation (CrCoDate _ completion) = CrCoDate creation completion
+setCreationDate creation _                       = CrDate creation
+
+-- | Set the completion date to a specific value.
+--
+-- The first argument is a default creation date, in case none exists.
+-- This is because a completion date can only exist in combination with a
+-- creation date, as per the todo.txt format.
+setCompletionDate :: Day -> Day -> Dates -> Dates
+setCompletionDate _ completion (CrCoDate creation _) = CrCoDate creation completion
+setCompletionDate creation completion _              = CrCoDate creation completion
+
+{- Parsing -}
+
+type Parser = Parsec Void String
+
+-- | Parse one date of the format @YYYY-MM-DD@ (with no leading or trailing spaces).
+day :: Parser Day
+day = label "date" $ do
+  y <- integer
+  void $ char '-'
+  m <- int
+  void $ char '-'
+  d <- int
+  pure $ fromGregorian y m d
+  where
+    integer :: Parser Integer
+    integer = read <$> count 4 digitChar
+    int :: Parser Int
+    int = read <$> count 2 digitChar
+
+-- | Parse either zero, one or two dates of the format @YYYY-MM-DD@ (with no leading or trailing spaces).
+--
+-- If only one date is present, it is interpreted as the creation date.
+dates :: Parser Dates
+dates = try datesCrCo <|> try datesCr <|> pure NoDate
+  where
+    datesCrCo :: Parser Dates
+    datesCrCo = CrCoDate <$> day <*> (char ' ' *> day)
+    datesCr :: Parser Dates
+    datesCr = CrDate <$> day
diff --git a/src/TaskMachine/Todotxt/Priority.hs b/src/TaskMachine/Todotxt/Priority.hs
new file mode 100644
index 0000000..cd72b33
--- /dev/null
+++ b/src/TaskMachine/Todotxt/Priority.hs
@@ -0,0 +1,57 @@
+-- | Read, parse and write files in the <https://github.com/todotxt/todo.txt todo.txt> format.
+
+
+module TaskMachine.Todotxt.Priority
+  ( Priority()
+  , priorityToChar
+  , charToPriority
+  , showPriority
+  -- * Parsing
+  , priority
+  ) where
+
+import           Control.Monad
+import           Data.List.NonEmpty
+import           Data.Void
+
+import           Data.Set              as Set
+import           Data.Time.Calendar
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+import           Text.Megaparsec.Error
+
+data Priority
+  = PrioA | PrioB | PrioC | PrioD | PrioE | PrioF | PrioG
+  | PrioH | PrioI | PrioJ | PrioK | PrioL | PrioM | PrioN
+  | PrioO | PrioP | PrioQ | PrioR | PrioS | PrioT | PrioU
+  | PrioV | PrioW | PrioX | PrioY | PrioZ
+  deriving (Bounded, Enum, Eq, Show, Ord)
+
+priorityToChar :: Priority -> Char
+priorityToChar p = toEnum (fromEnum 'A' + fromEnum p)
+
+charToPriority :: Char -> Maybe Priority
+charToPriority c
+  | min_value <= value && value <= max_value = Just $ toEnum value
+  | otherwise                                = Nothing
+  where
+    value = fromEnum c - fromEnum 'A'
+    min_value = fromEnum (minBound :: Priority)
+    max_value = fromEnum (maxBound :: Priority)
+
+showPriority :: Priority -> String
+showPriority p = '(' : priorityToChar p : ")"
+
+{- Parsing -}
+
+type Parser = Parsec Void String
+
+priorityChar :: Parser Priority
+priorityChar = do
+  c <- anyChar
+  case charToPriority c of
+    Just p -> pure p
+    Nothing -> failure (Just $ Tokens $ c :| []) (Set.singleton $ Label $ 'p' :| "riority character")
+
+priority :: Parser Priority
+priority = char '(' *> priorityChar <* char ')'
diff --git a/stack.yaml b/stack.yaml
index 3d20d3a..b0252a3 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -15,7 +15,7 @@
 # resolver:
 #  name: custom-snapshot
 #  location: "./custom-snapshot.yaml"
-resolver: lts-11.11
+resolver: lts-12.9
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.
@@ -40,6 +40,8 @@ packages:
 # Dependency packages to be pulled from upstream that are not in the resolver
 # (e.g., acme-missiles-0.3)
 # extra-deps: []
+extra-deps:
+- ConfigFile-1.1.4
 
 # Override default flag values for local packages and extra-deps
 # flags: {}