From 0436cda4efc15fd61af3b881b7fbac8d77747a77 Mon Sep 17 00:00:00 2001
From: Per Malmberg <per.malmberg@gmail.com>
Date: Tue, 22 May 2018 21:45:00 +0200
Subject: [PATCH] Simplified handling of clock changes

---
 libcron/Cron.h | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/libcron/Cron.h b/libcron/Cron.h
index b4101a0..6746ada 100644
--- a/libcron/Cron.h
+++ b/libcron/Cron.h
@@ -128,24 +128,37 @@ namespace libcron
         }
 
 
+
+
         if (first_tick)
         {
             first_tick = false;
         }
-        else if (now > last_tick && now - last_tick <= std::chrono::hours{3})
+        else
         {
-            // Reschedule all tasks.
-            for (auto& t : tasks.get_tasks())
+            // https://linux.die.net/man/8/cron
+
+            constexpr auto three_hours = std::chrono::hours{3};
+            auto diff = now - last_tick;
+            auto absolute_diff = diff > diff.zero() ? diff : -diff;
+
+            if(absolute_diff >= three_hours)
             {
-                t.calculate_next(now);
+                // Time changes of more than 3 hours are considered to be corrections to the
+                // clock or timezone, and the new time is used immediately.
+                for (auto& t : tasks.get_tasks())
+                {
+                    t.calculate_next(now);
+                }
             }
-        }
-        else if (now < last_tick && now - last_tick <= -std::chrono::hours{3})
-        {
-            // Reschedule all tasks.
-            for (auto& t : tasks.get_tasks())
+            else
             {
-                t.calculate_next(now);
+                // Change of less than three hours
+
+                // If time has moved backwards: Since tasks are not rescheduled, they won't run before
+                // we're back at least the original point in time which prevents running tasks twice.
+
+                // If time has moved forward, tasks that would have run since last tick will be run.
             }
         }