diff --git a/src/calibre/utils/formatter_functions.py b/src/calibre/utils/formatter_functions.py
index b8ff7b0387..2d0106c59a 100644
--- a/src/calibre/utils/formatter_functions.py
+++ b/src/calibre/utils/formatter_functions.py
@@ -1187,7 +1187,7 @@ class BuiltinDaysBetween(BuiltinFormatterFunction):
         except:
             return ''
         i = d1 - d2
-        return str('%d.%d'%(i.days, i.seconds/8640))
+        return '%.1f'%(i.days + (i.seconds/(24.0*60.0*60.0)))
 
 class BuiltinLanguageStrings(BuiltinFormatterFunction):
     name = 'language_strings'