Topics

Zulu time, GMT & UTC


Jerry N9AVY
 

This may clear up some confusion about time ...

Jerry  n9avy


"Zulutime, 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.


Fr Richard R
 

Thank you Jerry

Fr Richard
WB8YXF




On Thursday, December 10, 2020, 1:07:54 PM EST, Jerry N9AVY <n9avy@...> wrote:


This may clear up some confusion about time ...

Jerry  n9avy


"Zulutime, 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.


Rick - N7WE
 

I find the attached chart helpful.
--
Rick - N7WE
070 - #1602


Jerry N9AVY
 

It's a helpful chart, but it's easier to hand the GMT/UTC/Zulu time at 0000 hours in one's mind and start from there rather than having to refer to a chart.  Am sure chart will be helpful to many, but I wouldn't rely on it too heavily.

Once you know your time zone is it get easier.  For example:  CST is GMT/UTC plus 5 hours and CDST is plus 6 hours.  Simple, yes ?

Jerry  n9avy

On Thursday, December 10, 2020, 12:31:23 PM CST, Rick - N7WE <n7we1980@...> wrote:


I find the attached chart helpful.
--
Rick - N7WE
070 - #1602


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.


Josef 'Jeff' Sipek
 

On Thu, Dec 10, 2020 at 18:38:28 +0000, Jerry N9AVY wrote:
...
Once you know your time zone is it get easier.  For example:  CST is
GMT/UTC plus 5 hours and CDST is plus 6 hours.  Simple, yes ?
Assuming CST is Central Standard Time, I have two things to point out:

(1)

CST is UTC *minus* 6 hours
CDT is UTC *minus* 5 hours

(2) The official abbreviations for the timezones in the lower 48 states are:
EST/CST/MST/PST (winter/non-DST) and EDT/CDT/MDT/PDT (winter/DST)

Jeff.


Jerry  n9avy
On Thursday, December 10, 2020, 12:31:23 PM CST, Rick - N7WE <n7we1980@gmail.com> wrote:

I find the attached chart helpful.
--
Rick - N7WE
070 - #1602





Josef 'Jeff' Sipek
 

On Thu, Dec 10, 2020 at 13:58:41 -0500, Josef 'Jeff' Sipek wrote:
On Thu, Dec 10, 2020 at 18:38:28 +0000, Jerry N9AVY wrote:
...
Once you know your time zone is it get easier.  For example:  CST is
GMT/UTC plus 5 hours and CDST is plus 6 hours.  Simple, yes ?
Assuming CST is Central Standard Time, I have two things to point out:

(1)

CST is UTC *minus* 6 hours
CDT is UTC *minus* 5 hours

(2) The official abbreviations for the timezones in the lower 48 states are:
EST/CST/MST/PST (winter/non-DST) and EDT/CDT/MDT/PDT (winter/DST)
Err, copy & paste error - that should read:

(2) The official abbreviations for the timezones in the lower 48 states are:
EST/CST/MST/PST (winter/non-DST) and EDT/CDT/MDT/PDT (summer/DST)


Sorry,

Jeff.



Jeff.


Jerry  n9avy
On Thursday, December 10, 2020, 12:31:23 PM CST, Rick - N7WE <n7we1980@gmail.com> wrote:

I find the attached chart helpful.
--
Rick - N7WE
070 - #1602










Paula K7PAX #1739
 

Good to know about Iceland time. That could be very useful!

Paula K7PAX

On Dec 10, 2020, at 10:50 AM, Josef 'Jeff' Sipek <jeffpc@josefsipek.net> wrote:

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.






Barry VA7GEM
 

I keep m computer on local time and my software (HRD)
on UTC. No question about what time it is.


Leland Sly
 

👍

On Fri, Dec 11, 2020 at 4:35 PM Barry VA7GEM via groups.io <boat.anchor=yahoo.ca@groups.io> wrote:
I keep m computer on local time and my software (HRD)
on UTC. No question about what time it is.


Jerry N9AVY
 

Yup, most software will convert local time from computer to UTC/GMT/Zulu.  If your software doesn't do that, you were ripped off !

Emoji

Jerry



On Friday, December 11, 2020, 04:35:32 PM CST, Barry VA7GEM via groups.io <boat.anchor@...> wrote:


I keep m computer on local time and my software (HRD)
on UTC. No question about what time it is.


Don - KM4UDX
 

Wait, Iceland time always = UTC?
Why am I just hearing this now? 
This is great.
--
Don, km4udx, uBITX, and other obscure acronyms go here...