 
 void log_data() {
 
-    char buffer[17] ;
+    char buffer[26] ;
+    // timestamp
     int hh = hour();
     int mm = minute();
     int ss = second();
-    // time
-    sprintf(buffer,"%02d:%02d:%02d,",hh,mm,ss);
+	int yyyy = year();
+	int mm = month();
+	int dd = day();
+
+	// 18 char excel timestamp
+    sprintf(buffer,"%02d-%02d-%02d %02d:%02d:%02d,",yyyy,mm,dd,hh,mm,ss);
 
     int dht11_code = DHT11.read(DHT11_PIN);
     micro_delay(200);

       
    if(dht11_code != 0 ) {
        // error
        sprintf(buffer+9,"%7s","error");

    } else {
        sprintf(buffer+9,"%-03d,%-03d",DHT11.temperature,DHT11.humidity);
    }

    buffer[16] = '\0' ;
    
    // write to SD card
    dataFile.println(buffer);

#include <TimeAlarms.h>
#include <DS1307RTC.h>
#include <dht11.h>

#include <SPI.h>
#include <SD.h>
