Question about get “Source” response time

[FeiTian01]

Where should i write code to get the response time of "Source" ? If i write the code in the "Source" transformer , the channel will report an error.
my code in "Source" transformer :

var date = connectorMessage.getResponseDate().getTime();
var dateString = DateUtil.formatDate('yyyy-MM-dd HH:mm:ss', date);
logger.info(dateString);

Error Message:

Transformer error
ERROR MESSAGE: Error evaluating transformer
com.mirth.connect.server.MirthJavascriptTransformerException: 
CHANNEL:	TEST_WS
CONNECTOR:	sourceConnector
SCRIPT SOURCE:	TRANSFORMER
SOURCE CODE:	
118: }
119: }function doFilter() { phase[0] = 'filter'; return ((filterRule1() == true)); }
120: function doTransform() { phase[0] = 'transformer'; logger = Packages.org.apache.log4j.Logger.getLogger(phase[0]);
121: 
122: 
123: var date = connectorMessage.getResponseDate().getTime();
124: var dateString = DateUtil.formatDate('yyyy-MM-dd HH:mm:ss', date);
125: logger.info(dateString);
126: if ('xml' === typeof msg) {
127:     if (msg.hasSimpleContent()) {
LINE NUMBER:	123
DETAILS:	TypeError: Cannot call method "getTime" of null
	at fe0115da-a468-4d29-926b-a427260e0c6e:123 (doTransform)
	at fe0115da-a468-4d29-926b-a427260e0c6e:147 (doScript)
	at fe0115da-a468-4d29-926b-a427260e0c6e:149
	at com.mirth.connect.server.transformers.JavaScriptFilterTransformer$FilterTransformerTask.doCall(JavaScriptFilterTransformer.java:154)
	at com.mirth.connect.server.transformers.JavaScriptFilterTransformer$FilterTransformerTask.doCall(JavaScriptFilterTransformer.java:119)
	at com.mirth.connect.server.util.javascript.JavaScriptTask.call(JavaScriptTask.java:113)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)

[pacmano1]

[FeiTian01]

thank you very much @pacmano1