Http headers can be modified like this. I use this link as example. Since it's one of the Google's example.
Read it and find EndpointsAsyncTask you can use GoogleClientRequestInitializer for this. inside
public void initialize(AbstractGoogleClientRequest abstractGoogleClientRequest) throws IOException
you can set headers like this.
HttpHeaders headers = abstractGoogleClientRequest.getRequestHeaders();
headers.setDate(dateFormat.format(calendar.getTime()));
headers.set("MyCustomHeader", "HeaderValue");
abstractGoogleClientRequest.setRequestHeaders(headers);
In above code setDate() can be used to set Date header and set() method has been used to set my own header that can be read from server side. I have used calendar class to get date but Calendar class initialization is not shown here.
I have also write a note on my blog to show how it's actually reside in the method. if you like read that too.