Date Format for the response

I am checking the status of an outgoing fax by using this REST API

https://rest.interfax.net/outbound/faxes/:id

I am getting the date-time fields for submitted time as follows:

submitTime":"2016-02-23T17:45:55"
completionTime":"2016-02-23T17:46:31"

Where in the documentation it says that

Date-times are expected to be ISO 8601-compliant. See the link for details, but generally speaking, a date-time should look like this:

(date as YYYY-MM-DD)T(time)(GMT offset)

But, as you can see , I don't get any GMT offset

so is the GMT offset Zero?
and can I change the setting to return back the GMT offset?