diff --git a/hledger-lib/Hledger/Data/Dates.hs b/hledger-lib/Hledger/Data/Dates.hs
index 0e5d44fb2..23882e44d 100644
--- a/hledger-lib/Hledger/Data/Dates.hs
+++ b/hledger-lib/Hledger/Data/Dates.hs
@@ -892,7 +892,11 @@ lastthisnextthing = do
 
   return ("", T.unpack r, T.unpack p)
 
--- |
+-- | Parse a period expression, specifying a date span and optionally
+-- a reporting interval. Requires a reference "today" date for
+-- resolving any relative start/end dates (only; it is not needed for
+-- parsing the reporting interval).
+--
 -- >>> let p = parsePeriodExpr (parsedate "2008/11/26")
 -- >>> p "from Aug to Oct"
 -- Right (NoInterval,DateSpan 2008/08/01-2008/09/30)
@@ -944,6 +948,7 @@ periodexprp rdate = do
                     (,) NoInterval <$> periodexprdatespanp rdate
                    ]
 
+-- Parse a reporting interval and a date span.
 intervalanddateperiodexprp :: Day -> TextParser m (Interval, DateSpan)
 intervalanddateperiodexprp rdate = do
   i <- reportingintervalp
diff --git a/hledger-lib/Hledger/Reports/ReportOptions.hs b/hledger-lib/Hledger/Reports/ReportOptions.hs
index 6f7c64772..d07ff20b2 100644
--- a/hledger-lib/Hledger/Reports/ReportOptions.hs
+++ b/hledger-lib/Hledger/Reports/ReportOptions.hs
@@ -296,7 +296,9 @@ intervalFromRawOpts = lastDef NoInterval . catMaybes . map intervalfromrawopt
           either
             (\e -> usageError $ "could not parse period option: "++customErrorBundlePretty e)
             extractIntervalOrNothing $
-            parsePeriodExpr undefined (stripquotes $ T.pack v) -- reference date does not affect the interval
+            parsePeriodExpr
+              (error' "intervalFromRawOpts: did not expect to need today's date here") -- should not happen; we are just getting the interval, which does not use the reference date
+              (stripquotes $ T.pack v)
       | n == "daily"     = Just $ Days 1
       | n == "weekly"    = Just $ Weeks 1
       | n == "monthly"   = Just $ Months 1