diff --git a/hledger-lib/Hledger/Data/Dates.hs b/hledger-lib/Hledger/Data/Dates.hs
index 5f43af02c..b6c2129a2 100644
--- a/hledger-lib/Hledger/Data/Dates.hs
+++ b/hledger-lib/Hledger/Data/Dates.hs
@@ -561,6 +561,8 @@ justdatespan rdate = do
   d <- smartdate
   return $ spanFromSmartDate rdate d
 
+-- | Make a datespan from two valid date strings parseable by parsedate
+-- (or raise an error). Eg: mkdatespan "2011/1/1" "2011/12/31".
 mkdatespan :: String -> String -> DateSpan
 mkdatespan b = DateSpan (Just $ parsedate b) . Just . parsedate