Re: Zulu time, GMT & UTC


Josef 'Jeff' Sipek
 

On Thu, Dec 10, 2020 at 18:07:46 +0000, Jerry N9AVY wrote:
This may clear up some confusion about time ...
Jerry  n9avy

"Zulu" time, more commonly know as "GMT" ( Greenwich Mean Time )
is time at the Zero Meridian. ... Due to various scientific reasons and
increased accuracy in measuring the earth's rotation, a new timescale,
called Universal Time Coordinated or Coordinated Universal Time (UTC), has
been adopted and replaced the term GMT.
This is true. Additionally, using "GMT" can get *really* confusing, and
that's why I avoid it.

There is a subtle difference between the three quantities.

During the winter, UTC = GMT = "time in the UK" (well, +-1 second).

During the summer, things get more complicated. While UTC just continues on
without interruption, the "time in the UK" is BST (British Summer Time)
which equals UTC+1 hour. GMT is the confusing one. Some say that it is
equal to UTC, some say that it is equal to "time in the UK". (Obviously, it
cannot be both.)

It's very annoying to deal with, and that's why I strongly suggest everyone
avoids saying "GMT" - especially during the summer.

In short:

* if you mean "zulu" time, write "UTC"
* if you mean "time in the UK", just write it out. (E.g., "let's meet at
noon London-time" or if it is clear from the context: "noon local time")

Tip for scheduling cross-timezone meetings:

Make sure you agree on both the time and the timezone. Then, enter the
event into your calendar as that - even if it isn't your timezone. Any
calendaring program worth using will correctly deal with changes to DST. If
you try to be clever, you might end up showing up at the wrong time.

For example, say I living in Massachusetts want to schedule a QSO with
someone in Frankfurt, Germany. If we agree to 12:30 local time in
Frankfurt, I'll enter it into my calendar just like that: 12:30 in
Frankfurt. Then, it doesn't matter if it is DST here, summer time in EU, or
the weird in-between few weeks where the US is on DST but EU is still on
winter time.

If I try to be clever, and instead of 12:30 in Frankfurt I put 06:30 in
Massachusetts it may work most of the time (namely when US and EU are both
using winter/summer time). But during the spring and fall, when the local
time difference between MA and Germany is only 5 hours, I'll have to be on
the radio at 07:30. Had I entered it as 12:30 Frankfurt, the calendar would
have automatically figured it out. But if I enter it as 06:30 MA, the
calendar doesn't know it should adjust it by an hour.

Oh, and if you want to schedule something for a UTC time and your calendar
doesn't let you enter "zulu" time, pick Iceland as the timezone location.
Iceland uses UTC year-round as their local time.

Huh... this turned out longer than I anticipated. Anyway, calendaring is
hard, timezones are hard, and it is a minor miracle civilization hasn't
collapsed because of the two :)

Jeff.

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