diff --git a/include/Clock.h b/include/Clock.h
index 2ed163b..c78e7b5 100644
--- a/include/Clock.h
+++ b/include/Clock.h
@@ -16,7 +16,7 @@
 
 SNTPError clock_sync(NetworkInterface &net);
 
-const char *clock_timestring(const char *format);
+const char *clock_timestring(struct timeval time, const char *format);
 const char *clock_iso8601string(void);
 
 #endif
diff --git a/src/Clock.cpp b/src/Clock.cpp
index 93a1f8c..2c80205 100644
--- a/src/Clock.cpp
+++ b/src/Clock.cpp
@@ -28,8 +28,7 @@
 	return SNTPSuccess;
 }
 
-const char *clock_timestring(const char *format) {
-	struct timeval time = clock_time();
+const char *clock_timestring(struct timeval time, const char *format) {
 	struct tm *local_time = localtime(&time.tv_sec);
 
 	static char buffer[64];
@@ -46,5 +45,19 @@
 }
 
 const char *clock_iso8601string(void) {
-	return clock_timestring("%Y-%m-%dT%H:%M:%S");
+	struct timeval time = clock_time();
+	const char *timestring = clock_timestring(time, "%Y-%m-%dT%H:%M:%S");
+
+	static char buffer[128];
+	int buffer_size = sizeof(buffer);
+	int written;
+	written = snprintf(
+		buffer, buffer_size, "%s.%05li", timestring, time.tv_usec);
+
+	if (written >= buffer_size) {
+		// It overflowed, so return something useful
+		return "(iso8601 overflow)";
+	} else {
+		return buffer;
+	}
 }