diff --git a/cores/esp8266/HardwareSerial.h b/cores/esp8266/HardwareSerial.h
index ce9a1c566c..9d50be55e5 100644
--- a/cores/esp8266/HardwareSerial.h
+++ b/cores/esp8266/HardwareSerial.h
@@ -28,6 +28,7 @@
 #define HardwareSerial_h
 
 #include <inttypes.h>
+#include <time.h>
 #include "Stream.h"
 #include "uart.h"