How to convert Date to format "YYYY-MM-DDThh:mm:ss-hh:mm"?

I tried "yyyy-MM-dd'T'HH:mm:ss.SSSXXX" pattern and I am able to convert Date from "YYYY-MM-DD" to "YYYY-MM-DDThh:mm:ss-hh:mm" format as given below-

Date d = System.today();
Datetime myDT = datetime.newInstance(d.year(), d.month(),d.day());
String myDate = myDT.format('yyyy-MM-dd'T'HH:mm:ss.SSSXXX');
"YYYY-MM-DDThh:mm:ss-hh:mm" is ISO 8601 format. Check this link- https://www.w3.org/TR/xmlschema-2/#isoformats for ISO formats.
Refer http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html for formatting and parsing dates.