dojox/date/timezone consider DST in format

[J.J. Pianta]

Formatting a date for a particular timezone, we should take care of local timezone's DST.

Let's say we are GMT+2 and we need to format Sun, 26 Mar 2017 06:00:00 GMT+2 for GMT-4 (26 Mar 2017 at 00:00:00): we should roll our date by the difference between desired offset and local offset, to end up with Sun, 26 Mar 2017 00:00:00 GMT+2.

If our clock change for DST falls on Sun, 26 Mar 2017 02:00:00 GMT+2, we are rolling across the clock change and so we need to roll one hour less.