diff --git a/ChangeLog.txt b/ChangeLog.txt
index bdc53bfc..cd051b76 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -46,6 +46,7 @@ Clients:
   easily set for MQTT v5 clients.
 - Add `--random-filter` to mosquitto_sub, to allow only a certain proportion
   of received messages to be printed.
+- mosquitto_sub %j and %J timestamps are now in a ISO 8601 compatible format.
 
 
 1.6.9 - 20200227
diff --git a/client/sub_client_output.c b/client/sub_client_output.c
index e764ef21..6b4c6519 100644
--- a/client/sub_client_output.c
+++ b/client/sub_client_output.c
@@ -208,8 +208,9 @@ static int json_print_properties(cJSON *root, const mosquitto_property *properti
 #endif
 
 
-static int json_print(const struct mosquitto_message *message, const mosquitto_property *properties, const struct tm *ti, bool escaped, bool pretty)
+static int json_print(const struct mosquitto_message *message, const mosquitto_property *properties, const struct tm *ti, int ns, bool escaped, bool pretty)
 {
+	char buf[100];
 #ifdef WITH_CJSON
 	cJSON *root;
 	cJSON *tmp;
@@ -221,7 +222,11 @@ static int json_print(const struct mosquitto_message *message, const mosquitto_p
 		return MOSQ_ERR_NOMEM;
 	}
 
-	tmp = cJSON_CreateNumber(time(NULL));
+	strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S.000000Z%z", ti);
+	snprintf(&buf[strlen("2020-05-06T21:48:00.")], 9, "%06d", ns/1000);
+	buf[strlen("2020-05-06T21:48:00.000000")] = 'Z';
+
+	tmp = cJSON_CreateStringReference(buf);
 	if(tmp == NULL){
 		cJSON_Delete(root);
 		return MOSQ_ERR_NOMEM;
@@ -304,10 +309,12 @@ static int json_print(const struct mosquitto_message *message, const mosquitto_p
 	
 	return MOSQ_ERR_SUCCESS;
 #else
-	char buf[100];
+	strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S.000000Z%z", ti);
+	snprintf(&buf[strlen("2020-05-06T21:48:00.")], 9, "%06d", ns/1000);
+	buf[strlen("2020-05-06T21:48:00.000000")] = 'Z';
 
-	strftime(buf, 100, "%s", ti);
-	printf("{\"tst\":%s,\"topic\":\"%s\",\"qos\":%d,\"retain\":%d,\"payloadlen\":%d,", buf, message->topic, message->qos, message->retain, message->payloadlen);
+	tmp = cJSON_CreateStringReference(buf);
+	printf("{\"tst\":\"%s\",\"topic\":\"%s\",\"qos\":%d,\"retain\":%d,\"payloadlen\":%d,", buf, message->topic, message->qos, message->retain, message->payloadlen);
 	if(message->qos > 0){
 		printf("\"mid\":%d,", message->mid);
 	}
@@ -403,7 +410,7 @@ static void formatted_print(const struct mosq_config *lcfg, const struct mosquit
 								return;
 							}
 						}
-						if(json_print(message, properties, ti, true, lcfg->pretty) != MOSQ_ERR_SUCCESS){
+						if(json_print(message, properties, ti, ns, true, lcfg->pretty) != MOSQ_ERR_SUCCESS){
 							err_printf(lcfg, "Error: Out of memory.\n");
 							return;
 						}
@@ -416,7 +423,7 @@ static void formatted_print(const struct mosq_config *lcfg, const struct mosquit
 								return;
 							}
 						}
-						rc = json_print(message, properties, ti, false, lcfg->pretty);
+						rc = json_print(message, properties, ti, ns, false, lcfg->pretty);
 						if(rc == MOSQ_ERR_NOMEM){
 							err_printf(lcfg, "Error: Out of memory.\n");
 							return;
diff --git a/man/mosquitto_rr.1.xml b/man/mosquitto_rr.1.xml
index 683e212f..b4f25ccb 100644
--- a/man/mosquitto_rr.1.xml
+++ b/man/mosquitto_rr.1.xml
@@ -728,13 +728,13 @@
 				<listitem><para><option>%j</option> JSON output of message
 						parameters and timestamp, with a quoted and escaped
 						payload. For example
-						<code>{"tst":1470825369,"topic":"greeting","qos":0,"retain":0,"payload":"hello
+						<code>{"tst":"2020-05-06T22:12:00.000000+0100","topic":"greeting","qos":0,"retain":0,"payload":"hello
 							world"}</code></para></listitem>
 				<listitem><para><option>%J</option> JSON output of message
 						parameters and timestamp, with a non-quoted and
 						non-escaped payload - this means the payload must
 						itself be valid JSON. For example:
-						<code>{"tst":1470825369,"topic":"foo","qos":0,"retain":0,"payload":{"temperature":27.0,"humidity":57}}</code>.</para>
+						<code>{"tst":"2020-05-06T22:12:00.000000+0100","topic":"foo","qos":0,"retain":0,"payload":{"temperature":27.0,"humidity":57}}</code>.</para>
 					<para>If the payload is not valid JSON, then the error message "Error: Message payload is not valid JSON on topic
 						&lt;topic&gt;" will be printed to stderr.</para></listitem>
 				<listitem><para><option>%I</option> ISO-8601 format date and time, e.g. 2016-08-10T09:47:38+0100</para></listitem>
diff --git a/man/mosquitto_sub.1.xml b/man/mosquitto_sub.1.xml
index 841851cc..8419197b 100644
--- a/man/mosquitto_sub.1.xml
+++ b/man/mosquitto_sub.1.xml
@@ -857,13 +857,13 @@ mosquitto_sub -t 'bbc/#' -T bbc/bbc1 --remove-retained</programlisting>
 				<listitem><para><option>%j</option> JSON output of message
 						parameters and timestamp, with a quoted and escaped
 						payload. For example
-						<code>{"tst":1470825369,"topic":"greeting","qos":0,"retain":0,"payload":"hello
+						<code>{"tst":"2020-05-06T22:12:00.000000+0100","topic":"greeting","qos":0,"retain":0,"payload":"hello
 							world"}</code></para></listitem>
 				<listitem><para><option>%J</option> JSON output of message
 						parameters and timestamp, with a non-quoted and
 						non-escaped payload - this means the payload must
 						itself be valid JSON. For example:
-						<code>{"tst":1470825369,"topic":"foo","qos":0,"retain":0,"payload":{"temperature":27.0,"humidity":57}}</code>.</para>
+						<code>{"tst":"2020-05-06T22:12:00.000000+0100","topic":"foo","qos":0,"retain":0,"payload":{"temperature":27.0,"humidity":57}}</code>.</para>
 					<para>If the payload is not valid JSON, then the error message "Error: Message payload is not valid JSON on topic
 						&lt;topic&gt;" will be printed to stderr.</para></listitem>