diff --git a/lib/Rtc_Pcf8563/Rtc_Pcf8563.cpp b/lib/Rtc_Pcf8563/Rtc_Pcf8563.cpp
index 4338538..9c508b5 100644
--- a/lib/Rtc_Pcf8563/Rtc_Pcf8563.cpp
+++ b/lib/Rtc_Pcf8563/Rtc_Pcf8563.cpp
@@ -804,7 +804,7 @@ uint64_t Rtc_Pcf8563::getTimestamp64()
   timestamp *= 86400; // convert days in seconds
 
   // convert time to second and add it to timestamp
-  unsigned long timeTemp = hour * 60 + minute;
+  uint64_t timeTemp = hour * 60 + minute;
   timeTemp *= 60;
   timeTemp += sec;