diff --git a/clock.cpp b/clock.cpp
index e98857a..881cd71 100644
--- a/clock.cpp
+++ b/clock.cpp
@@ -10,7 +10,7 @@
 
 Clock::Clock(ClockSettings const& settings):
     m_settings(settings),
-    m_last_NTP_check(0),
+    m_next_NTP_check(0),
     m_NTP_waiting_for_response(false),
     m_UDP(),
     m_NTP_ip(),
@@ -65,14 +65,8 @@ void Clock::ntp_update()
     bool need_to_ask = false;
     if(timeStatus() != timeSet)
         need_to_ask = !m_NTP_waiting_for_response;
-    else if(m_NTP_waiting_for_response)
-    {
-        //5 minutes timeout for the NTP response
-        if((now() - m_last_NTP_check) > 5*60)
-            m_NTP_waiting_for_response = false;
-    }
     else
-        need_to_ask = (now() - m_last_NTP_check) > m_settings.cooldown();
+        need_to_ask = now() > m_next_NTP_check;
 
     if(need_to_ask) ntp_ask();
     ntp_checkResponse();
@@ -100,7 +94,7 @@ void Clock::ntp_ask()
     m_UDP.write(m_NTP_buffer, NTP_BUFFER_SIZE);
     m_UDP.endPacket();
 
-    m_last_NTP_check = now();
+    m_next_NTP_check = now() + NTP_RETRY_COOLDOWN;
     m_NTP_waiting_for_response = true;
 }
 
@@ -121,7 +115,7 @@ void Clock::ntp_checkResponse()
 
     time_t UNIX_time = NTP_time - 2208988800UL;
 
-    m_last_NTP_check = UNIX_time;
+    m_next_NTP_check = UNIX_time + m_settings.cooldown();
     RTC.set(UNIX_time);
     setTime(UNIX_time);
 
diff --git a/clock.h b/clock.h
index 13a896e..4f711a9 100644
--- a/clock.h
+++ b/clock.h
@@ -7,6 +7,7 @@
 
 unsigned int const NTP_BUFFER_SIZE = 48;
 unsigned int const NTP_PORT = 123;
+unsigned int const NTP_RETRY_COOLDOWN = 5*60;
 unsigned int const TZ_RETRY_COOLDOWN = 60;
 
 class Clock
@@ -31,7 +32,7 @@ class Clock
     protected:
         ClockSettings const& m_settings;
 
-        time_t    m_last_NTP_check;
+        time_t    m_next_NTP_check;
         bool      m_NTP_waiting_for_response;
 
         WiFiUDP   m_UDP;