[phlo]

> If you ask for output in EDT, but the date is in December (which is not part of daylight savings time) then should the date output be UTC-4 or UTC-5?

EST is always UTC-4 and EDT is always UTC-5. Both of these exist throughout the year. The only change happening in Spring/Autumn is that some places change which time zone they currently observe.

This also addresses the second problem that you outlined. As long as the query is properly specified (if you want information related to a location, use America/New York; if you want information related to a time zone, use the time zone), you can always get an unambiguous and correct answer.

[dmm]

To be pedantic EST and EDT are offsets. America/New_York is the timezone which consists of multiple offsets that depends on the time of year and date mostly determined by the local civil authority. For example in 2005, the US extended daylight saving time by four weeks. This difference between 2004 and 2006 is a part of the timezone.

[phlo]

Cool, I hadn't realized that distinction in terminology. Thanks for pointing it out! :)

[throw0101a]

> EST is always UTC-4 and EDT is always UTC-5.

Except you have those reversed. :)

[emmelaich]

< EST is always UTC-4

Unfortunately not. Australia also used EST until recently. Now they've introduced AEST, but I'm sure there's systems out there using EST.

[mikelward]

When somebody writes EST in summer they mean UTC-4. Interpreting that as UTC-5 is almost always wrong.

[kelnos]

Right, but that's because people are misinformed as to how timezones work. EST means UTC-5; people who refer to EST as current during the summer are incorrect. Most news outlets avoid the problem by just saying "ET" or "Eastern Time", which I wish more people would do.