diff --git a/hledger-lib/Hledger/Data/Account.hs b/hledger-lib/Hledger/Data/Account.hs
index f24f7140d..69898d19f 100644
--- a/hledger-lib/Hledger/Data/Account.hs
+++ b/hledger-lib/Hledger/Data/Account.hs
@@ -387,7 +387,7 @@ tests_Account = testGroup "Account" [
       testCase "no postings, no days" $
         accountFromPostings undefined [] @?= accountTree "root" []
      ,testCase "no postings, only 2000-01-01" $
-         allAccounts (all (\d -> (ModifiedJulianDay $ toInteger d) == fromGregorian 2000 01 01) . M.keys . pdperiods . adata)
+         allAccounts (all (== fromGregorian 2000 01 01) . M.keys . pdperiods . adata)
                      (accountFromPostings undefined []) @? "Not all adata have exactly 2000-01-01"
     ]
   ]
diff --git a/hledger-lib/Hledger/Data/DayPartition.hs b/hledger-lib/Hledger/Data/DayPartition.hs
index 96d8da8aa..48e38df36 100644
--- a/hledger-lib/Hledger/Data/DayPartition.hs
+++ b/hledger-lib/Hledger/Data/DayPartition.hs
@@ -128,7 +128,7 @@ unionDayPartitions (DayPartition (PeriodData h as)) (DayPartition (PeriodData h'
 dayPartitionStartEnd :: DayPartition -> (Day, Day)
 dayPartitionStartEnd (DayPartition (PeriodData _ ds)) =
   -- Guaranteed not to error because the IntMap is non-empty.
-  (intToDay . fst $ M.findMin ds, snd $ M.findMax ds)
+  (fst $ M.findMin ds, snd $ M.findMax ds)
 
 -- | Find the start and end dates of the period within a 'DayPartition' which contains a given day.
 -- If the day is after the end of the last period, it is assumed to be within the last period.
@@ -241,8 +241,6 @@ intervalBoundaryBefore i d =
     Just ((start, _) :| _ ) -> start
     _ -> d
 
-intToDay = ModifiedJulianDay . toInteger
-
 
 -- tests:
 
diff --git a/hledger-lib/Hledger/Data/Json.hs b/hledger-lib/Hledger/Data/Json.hs
index 691e2a49c..9a204398e 100644
--- a/hledger-lib/Hledger/Data/Json.hs
+++ b/hledger-lib/Hledger/Data/Json.hs
@@ -26,7 +26,6 @@ import           Data.Maybe (fromMaybe)
 import Data.Text.Lazy qualified    as TL
 import Data.Text.Lazy.Builder qualified as TB
 import Data.Map qualified as M
-import Data.Time (Day (..))
 import           Text.Megaparsec (Pos, SourcePos, mkPos, unPos)
 
 import           Hledger.Data.Types
@@ -161,7 +160,7 @@ instance ToJSON BalanceData
 instance ToJSON a => ToJSON (PeriodData a) where
   toJSON a = object
     [ "pdpre" .= pdpre a
-    , "pdperiods" .= map (\(d, x) -> (ModifiedJulianDay (toInteger d), x)) (M.toList $ pdperiods a)
+    , "pdperiods" .= (M.toList $ pdperiods a)
     ]
 
 instance ToJSON a => ToJSON (Account a) where
@@ -227,7 +226,7 @@ instance FromJSON BalanceData
 instance FromJSON a => FromJSON (PeriodData a) where
   parseJSON = withObject "PeriodData" $ \v -> PeriodData
     <$> v .: "pdpre"
-    <*> (M.fromList . map (\(d, x) -> (fromInteger $ toModifiedJulianDay d, x)) <$> v .: "pdperiods")
+    <*> (M.fromList <$> v .: "pdperiods")
 
 -- XXX The ToJSON instance replaces subaccounts with just names.
 -- Here we should try to make use of those to reconstruct the
diff --git a/hledger-lib/Hledger/Data/PeriodData.hs b/hledger-lib/Hledger/Data/PeriodData.hs
index 40092e85c..c924196c2 100644
--- a/hledger-lib/Hledger/Data/PeriodData.hs
+++ b/hledger-lib/Hledger/Data/PeriodData.hs
@@ -26,7 +26,6 @@ import Data.Foldable1 (Foldable1(..))
 #else
 import Control.Applicative (liftA2)
 #endif
-import Data.Bifunctor (first)
 #if !MIN_VERSION_base(4,20,0)
 import Data.List (foldl')
 #endif
@@ -44,7 +43,7 @@ instance Show a => Show (PeriodData a) where
         showString "PeriodData"
       . showString "{ pdpre = " . shows h
       . showString ", pdperiods = "
-      . showString "fromList " . shows (map (\(day, x) -> (intToDay day, x)) $ M.toList ds)
+      . showString "fromList " . shows (M.toList ds)
       . showChar '}'
 
 instance Foldable PeriodData where
@@ -73,17 +72,17 @@ instance Monoid a => Monoid (PeriodData a) where
 
 -- | Construct a 'PeriodData' from a historical data value and a list of (period start, period data) pairs.
 periodDataFromList :: a -> [(Day, a)] -> PeriodData a
-periodDataFromList h = PeriodData h . M.fromList . map (\(d, a) -> (dayToInt d, a))
+periodDataFromList h = PeriodData h . M.fromList
 
 -- | Convert 'PeriodData' to a historical data value and a list of (period start, period data) pairs.
 periodDataToList :: PeriodData a -> (a, [(Day, a)])
-periodDataToList (PeriodData h as) = (h, map (\(s, e) -> (intToDay s, e)) $ M.toList as)
+periodDataToList (PeriodData h as) = (h, M.toList as)
 
 -- | Get the data for the period containing the given 'Day', and that period's start date.
 -- If the day is after the end of the last period, it is assumed to be within the last period.
 -- If the day is before the start of the first period (ie, in the historical period), return Nothing.
 lookupPeriodData :: Day -> PeriodData a -> Maybe (Day, a)
-lookupPeriodData d (PeriodData _ as) = first intToDay <$> M.lookupLE (dayToInt d) as
+lookupPeriodData d (PeriodData _ as) = M.lookupLE d as
 
 -- | Get the data for the period containing the given 'Day', and that period's start date.
 -- If the day is after the end of the last period, it is assumed to be within the last period.
@@ -98,7 +97,7 @@ lookupPeriodDataOrHistorical d pd@(PeriodData h _) = case lookupPeriodData d pd
 insertPeriodData :: Semigroup a => Maybe Day -> a -> PeriodData a -> PeriodData a
 insertPeriodData mday b balances = case mday of
     Nothing  -> balances{pdpre = pdpre balances <> b}
-    Just day -> balances{pdperiods = M.insertWith (<>) (dayToInt day) b $ pdperiods balances}
+    Just day -> balances{pdperiods = M.insertWith (<>) day b $ pdperiods balances}
 
 -- | Merge two 'PeriodData', using the given operation to combine their data values.
 --
@@ -119,8 +118,6 @@ mergePeriodData only1 only2 f = \(PeriodData h1 as1) (PeriodData h2 as2) ->
 padPeriodData :: a -> PeriodData b -> PeriodData a -> PeriodData a
 padPeriodData x pad bal = bal{pdperiods = pdperiods bal <> (x <$ pdperiods pad)}
 
-intToDay = ModifiedJulianDay . toInteger
-dayToInt = fromInteger . toModifiedJulianDay
 
 -- tests
 
diff --git a/hledger-lib/Hledger/Data/Types.hs b/hledger-lib/Hledger/Data/Types.hs
index 0924da309..80725316b 100644
--- a/hledger-lib/Hledger/Data/Types.hs
+++ b/hledger-lib/Hledger/Data/Types.hs
@@ -754,10 +754,9 @@ data Account a = Account {
 -- The report periods are typically all the same length, but need not be.
 --
 -- Report periods are represented only by their start dates, used as the keys of a Map.
--- (As integers, like the one inside the Day type, representing days before/after 1858-11-17.)
 data PeriodData a = PeriodData {
-   pdpre     :: a                -- ^ data for the period before the report
-  ,pdperiods :: M.Map Integer a  -- ^ data for each period within the report
+   pdpre     :: a            -- ^ data for the period before the report
+  ,pdperiods :: M.Map Day a  -- ^ data for each period within the report
   } deriving (Eq, Ord, Functor, Generic)
 
 -- | Data that's useful in "balance" reports:
diff --git a/hledger-lib/Hledger/Reports/MultiBalanceReport.hs b/hledger-lib/Hledger/Reports/MultiBalanceReport.hs
index 49ec671bd..3b9cf91ff 100644
--- a/hledger-lib/Hledger/Reports/MultiBalanceReport.hs
+++ b/hledger-lib/Hledger/Reports/MultiBalanceReport.hs
@@ -454,7 +454,7 @@ makeMultiBalanceReportRow = makePeriodicReportRow nullmixedamt sumAndAverageMixe
 -- | Build a report row.
 --
 -- Calculate the column totals. These are always the sum of column amounts.
-makePeriodicReportRow :: c -> (M.Map Integer c -> (c, c))
+makePeriodicReportRow :: c -> (M.Map Day c -> (c, c))
                       -> ReportOpts -> (b -> c)
                       -> a -> Account b -> PeriodicReportRow a c
 makePeriodicReportRow nullEntry totalAndAverage ropts balance name acct =