diff --git a/README.md b/README.md
index 1cf6607c..dc9bae6c 100644
--- a/README.md
+++ b/README.md
@@ -256,12 +256,12 @@ The data is written to the card and after 3 write-operations the data is flushed
 
 And finally: this is the data written to the disk:
 
-    date, time, wifi, bluet
-    00.00.1970,00:01:09,2,0
-    00.00.1970,00:02:09,1,0
-    00.00.1970,00:03:09,2,0
-
-Format of the data is CSV, which can easily imported into LibreOffice, Excel, .....
+	timestamp, wifi, ble
+	2022-01-28T19:36:35Z,17,48
+	2022-01-28T19:37:35Z,21,52
+	2022-01-28T19:38:35Z,14,49
+	
+Format of the data is CSV, timestamp is ISO8601, which can easily imported into LibreOffice, Excel, Influx, etc.
 
 If you want to change this please look into src/sdcard.cpp and include/sdcard.h.
 
diff --git a/include/sdcard.h b/include/sdcard.h
index 5c34dc11..8d20ced5 100644
--- a/include/sdcard.h
+++ b/include/sdcard.h
@@ -55,7 +55,7 @@
 #endif
 
 #define SDCARD_FILE_NAME "/paxcount.%02d"
-#define SDCARD_FILE_HEADER "date, time, wifi, bluet"
+#define SDCARD_FILE_HEADER "timestamp, wifi, ble"
 
 #if (COUNT_ENS)
 #define SDCARD_FILE_HEADER_CWA ",cwa"
diff --git a/src/sdcard.cpp b/src/sdcard.cpp
index 925ab207..783cf657 100644
--- a/src/sdcard.cpp
+++ b/src/sdcard.cpp
@@ -42,11 +42,10 @@ bool sdcard_init() {
 void sdcardWriteData(uint16_t noWifi, uint16_t noBle,
                      __attribute__((unused)) uint16_t noBleCWA) {
   static int counterWrites = 0;
-  char tempBuffer[12 + 1];
+  char tempBuffer[20 + 1];
   time_t t = time(NULL);
   struct tm tt;
-  localtime_r(&t, &tt);
-  mktime(&tt);
+  gmtime_r(&t, &tt); // make UTC timestamp
 
 #if (HAS_SDS011)
   sdsStatus_t sds;
@@ -56,19 +55,17 @@ void sdcardWriteData(uint16_t noWifi, uint16_t noBle,
     return;
 
   ESP_LOGD(TAG, "writing to SD-card");
-  strftime(tempBuffer, sizeof(tempBuffer), "%d.%m.%Y", &tt);
+  strftime(tempBuffer, sizeof(tempBuffer), "%FT%TZ", &tt);
   fileSDCard.print(tempBuffer);
-  strftime(tempBuffer, sizeof(tempBuffer), "%H.%M.%S", &tt);
-  fileSDCard.print(tempBuffer);
-  sprintf(tempBuffer, "%d,%d", noWifi, noBle);
+  snprintf(tempBuffer, sizeof(tempBuffer), ",%d,%d", noWifi, noBle);
   fileSDCard.print(tempBuffer);
 #if (COUNT_ENS)
-  sprintf(tempBuffer, ",%d", noBleCWA);
+  snprintf(tempBuffer, sizeof(tempBuffer), ",%d", noBleCWA);
   fileSDCard.print(tempBuffer);
 #endif
 #if (HAS_SDS011)
   sds011_store(&sds);
-  sprintf(tempBuffer, ",%5.1f,%4.1f", sds.pm10, sds.pm25);
+  snprintf(tempBuffer, sizeof(tempBuffer), ",%5.1f,%4.1f", sds.pm10, sds.pm25);
   fileSDCard.print(tempBuffer);
 #endif
   fileSDCard.println();
@@ -88,7 +85,7 @@ void createFile(void) {
 
   for (int i = 0; i < 100; i++) {
     sprintf(bufferFilename, SDCARD_FILE_NAME, i);
-    // ESP_LOGD(TAG, "SD: looking for file <%s>", bufferFilename);
+    ESP_LOGD(TAG, "SD: looking for file <%s>", bufferFilename);
 
 #if HAS_SDCARD == 1
     bool fileExists = SD.exists(bufferFilename);
@@ -97,7 +94,7 @@ void createFile(void) {
 #endif
 
     if (!fileExists) {
-      // ESP_LOGD(TAG, "SD: file does not exist: opening");
+      ESP_LOGD(TAG, "SD: file does not exist: creating");
 
 #if HAS_SDCARD == 1
       fileSDCard = SD.open(bufferFilename, FILE_WRITE);
@@ -106,7 +103,7 @@ void createFile(void) {
 #endif
 
       if (fileSDCard) {
-        ESP_LOGD(TAG, "SD: name opened: <%s>", bufferFilename);
+        ESP_LOGD(TAG, "SD: file opened: <%s>", bufferFilename);
         fileSDCard.print(SDCARD_FILE_HEADER);
 #if (COUNT_ENS)
         fileSDCard.print(SDCARD_FILE_HEADER_CWA); // for Corona-data (CWA)
diff --git a/src/senddata.cpp b/src/senddata.cpp
index 5a310761..fce1d325 100644
--- a/src/senddata.cpp
+++ b/src/senddata.cpp
@@ -65,18 +65,6 @@ void SendPayload(uint8_t port) {
   mqtt_enqueuedata(&SendBuffer);
 #endif
 
-// write data to sdcard, if present
-#if (HAS_SDCARD)
-  if (port == COUNTERPORT) {
-    sdcardWriteData(libpax_macs_wifi, libpax_macs_ble
-#if (COUNT_ENS)
-                    ,
-                    cwa_report()
-#endif
-    );
-  }
-#endif
-
 } // SendPayload
 
 // interrupt triggered function to prepare payload to send
@@ -130,7 +118,16 @@ void sendData() {
 #ifdef HAS_DISPLAY
         dp_plotCurve(libpax_macs_ble + libpax_macs_wifi, true);
 #endif
-        break;
+#if (HAS_SDCARD)
+        sdcardWriteData(libpax_macs_wifi, libpax_macs_ble
+#if (COUNT_ENS)
+                        ,
+                        cwa_report()
+#endif
+        );
+#endif // HAS_SDCARD
+
+        break; // case COUNTDATA
 #endif
 
 #if (HAS_BME)