From 924831f3e40a5ab1299745539fb43836f657e095 Mon Sep 17 00:00:00 2001
From: SJost <jost@tcs.ifi.lmu.de>
Date: Tue, 5 Feb 2019 23:25:18 +0100
Subject: [PATCH] Fixes #262

---
 src/Handler/Sheet.hs          |  2 +-
 src/Handler/Utils/DateTime.hs | 12 +++++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/Handler/Sheet.hs b/src/Handler/Sheet.hs
index 4692468ea..f4031f7c9 100644
--- a/src/Handler/Sheet.hs
+++ b/src/Handler/Sheet.hs
@@ -447,7 +447,7 @@ getSheetNewR tid ssh csh = do
   now <- liftIO getCurrentTime
   let template = case lastSheets of
         ((Entity {entityVal=Sheet{..}}):_)  ->
-          let addTime = addWeeks $ max 1 $ succ $ weekDiff sheetActiveTo now
+          let addTime = addWeeks $ max 1 $ weeksToAdd sheetActiveTo now
           in Just $ SheetForm
             { sfName         = stepTextCounterCI sheetName
             , sfDescription  = sheetDescription
diff --git a/src/Handler/Utils/DateTime.hs b/src/Handler/Utils/DateTime.hs
index 69ace7b97..6e4bbb027 100644
--- a/src/Handler/Utils/DateTime.hs
+++ b/src/Handler/Utils/DateTime.hs
@@ -6,7 +6,7 @@ module Handler.Utils.DateTime
   , validDateTimeFormats, dateTimeFormatOptions
   , formatTimeMail
   , addOneWeek, addWeeks
-  , weekDiff
+  , weekDiff, weeksToAdd
   ) where
 
 import Import
@@ -150,5 +150,15 @@ weekDiff old new = dayDiff `div` 7
     dayNew  = utctDay new
     dayDiff = diffDays dayNew dayOld
 
+weeksToAdd :: UTCTime -> UTCTime -> Integer
+-- ^ Number of weeks needed to add so that first
+--   time occurs later than second time
+--   (loop avoids off-by-one error with weekDiff corner cases)
+weeksToAdd old new = loop 0 old
+  where
+    loop n t
+      | t > new   = n
+      | otherwise = loop (succ n) (addOneWeek t)
+
 
 -- addOneTerm? -> Move Handler.Utils.DateTime