Calculates and returns a time offset value to reflect the time difference
between your computer's clock and the current time according to the 'Date'
header in the given HTTP response, likely provided by a service endpoint
whose time you wish to treat as authoritative.
Ideally you should not rely on this method to overcome clock-related
disagreements between your computer and a service endpoint.
If you computer is set to update its clock periodically and has the
correct timezone setting you should never have to resort to this work-around.