vista and GMT

[pbx]

Vista seems confused about what GMT is:


my date/time is 3/27/2007 5:14PM in GMT -8 Pacific Time (US & Canada)


if I set the additional clock to both available GMTs:


GMT (Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London)
and
GMT (Casablanca, Monrovia, Reykjavik)


the first one (london) says its: 3/28/2007 1:14AM


the second one (casablanca) says its: 3/28/2007 12:14AM


the second one is right!


yikes!

 

[ray]

Vista seems confused about what GMT is:


my date/time is 3/27/2007 5:14PM in GMT -8 Pacific Time (US & Canada)


if I set the additional clock to both available GMTs:


GMT (Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London)
and
GMT (Casablanca, Monrovia, Reykjavik)


the first one (london) says its: 3/28/2007 1:14AM


the second one (casablanca) says its: 3/28/2007 12:14AM


the second one is right!


yikes!

I'm not terribly knowledgeable about such things, but could it be related
to UK being on DST?

 

[Tom Porterfield]

Vista seems confused about what GMT is:

my date/time is 3/27/2007 5:14PM in GMT -8 Pacific Time (US & Canada)

if I set the additional clock to both available GMTs:

GMT (Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London)
and
GMT (Casablanca, Monrovia, Reykjavik)

the first one (london) says its: 3/28/2007 1:14AM

the second one (casablanca) says its: 3/28/2007 12:14AM

the second one is right!

Actually they are both right but you have to understand what you are seeing.
Pacific Time (GMT-8) is currently in daylight savings time meaning you are
actually currently GMT-7. True GMT (Greenwich Mean Time) does not have a
concept of daylight savings time so GMT-8 is always GMT-8 even if when
adjusted it is GMT-7. Casablanca is not true GMT and while Casablanca does
not change its clock for daylight savings time, any conversion from local
time where DST is in effect does need to be accounted for and Windows does
this automacitally for you. To see the correct time for true Greenwich Mean
Time you need to set that as your timezone AND disable the automatic
adjustment for daylight savings time.

 

[pbx]

Actually they are both right but you have to understand what you are seeing.
Pacific Time (GMT-8) is currently in daylight savings time meaning you are
actually currently GMT-7. True GMT (Greenwich Mean Time) does not have a
concept of daylight savings time so GMT-8 is always GMT-8 even if when
adjusted it is GMT-7. Casablanca is not true GMT and while Casablanca does
not change its clock for daylight savings time, any conversion from local
time where DST is in effect does need to be accounted for and Windows does
this automacitally for you. To see the correct time for true Greenwich Mean
Time you need to set that as your timezone AND disable the automatic
adjustment for daylight savings time.

Tom--

that's a pretty good explanation but...

are saying that GMT is now +1 in London because London "sprang
forward" to summer time.

but then is:

http://wwp.greenwichmeantime.com/

wrong?

is there no absolute concept of GMT? http://wwp.greenwichmeantime.com/
matches the Casablanca setting.
Also the .NET Framework 2.0 (DateTime.UtcNow) running on the same
Vista machine reports GMT to match the Casablanca setting.

seems to me that GMT is GMT and London is now +1 GMT... so if the
Vista setting was London it would be correct... but it starts off
saying that its GMT (or is GMT <> UTC?)

chime in all (pun intended).

 

[Guest]

Technically windows does not use GMT (working out the time by looking at
stars etc). It uses UTC which is atomic clocks. The label of the time
changes with Daylight Saving. Windows does not use the label except in some
americian time zones. We just changed from AEDT to AEST.

 

[pbx]

Technically windows does not use GMT (working out the time by looking at
stars etc). It uses UTC which is atomic clocks. The label of the time
changes with Daylight Saving. Windows does not use the label except in some









- Show quoted text -

seems to me that GMT should not be shown as a "varying" time based on
daylight savings/summer time, but should always match UTC.
if Microsoft doesn't think this is a bug, then i would suggest that
they add a UTC option to the clocks timezone so those of us who want
to know UTC aka "GMT" can set the clock to it.

 

[Guest]

I'll be blunter.Microsoft lists TIME ZONES (those things composed of
daylight saving - so two places on the same longitude may have different
timezones) not names of times. GMT always = GMT = UTC (well to microseconds
or something anyway)

 

[Tom Porterfield]

that's a pretty good explanation but...

are saying that GMT is now +1 in London because London "sprang
forward" to summer time.

Well, yes and no. GMT is GMT and does not change with DST or what they call
Summer Time. But the GMT+-0 timezone for London does change. It is
important to make the distiction that the GMT timezone for London is not
GMT.

but then is:

http://wwp.greenwichmeantime.com/

wrong?

No, it is correct. But remember, during Summer time, the time in Greenwich
or London is not GMT but actually GMT(+1). Vista is showing timezones, not
GMT. So when you select the timezone for Greenwich, it applies the DST or
Summer Time offset when adjusting the clock. Since Casablanca does not
switch to DST or Summer Time, that is why it's GMT is an hour different than
Greenwich's GMT.

is there no absolute concept of GMT? http://wwp.greenwichmeantime.com/
matches the Casablanca setting.
Also the .NET Framework 2.0 (DateTime.UtcNow) running on the same
Vista machine reports GMT to match the Casablanca setting.

That is because .NET is using the GMT time, not the timezone for Greenwich.
In other words, it doesn't worry about applying any DST offset to the base
since true GMT never changes in that manner. It does apply the DST offset
to your local timezone.

seems to me that GMT is GMT and London is now +1 GMT... so if the
Vista setting was London it would be correct... but it starts off
saying that its GMT (or is GMT <> UTC?)

You've got it. The only thing I'll add is that what it is showing is the
GMT offset, not GMT. And since for London there is no offset it just shows
GMT rather than GMT-8 or similar. But that displayed offset doesn't include
any variance for seasonal clock changes. So for your current Pacific
timezone you're not really currently GMT-8 but more appropriately GMT-8(+1).
And London is currently GMT(+1). Casablanca is currently GMT. The display
doesn't change based on seasonal time changes because that is not how
timezones are defined, but it is something that has to be taken into
consideration when calculating the difference in time between two timezones.

 

[Les Hellawell]

Well, yes and no. GMT is GMT and does not change with DST or what they call
Summer Time. But the GMT+-0 timezone for London does change. It is
important to make the distiction that the GMT timezone for London is not
GMT.

Some slight confusion there! GMT is GMT whether in Greenwich (part
of London) or anywhere else. Clocks in London however, are not
currently set to GMT. We only set our clocks to GMT in winter.

Here in the UK our clocks are now set to BST (British Summer Time)
which is GMT +1.

Thus I have Vista set to GMT and to adjust auromatically for, what
M$ calls, "Daylight Savings time", which, fortunately, happens to
automatically set Vista to show BST in summer.


Les Hellawell
Greetings from

YORKSHIRE - The White Rose County

 

[Guest]

My recommendation is don't ever drive through Indiana.

 

[pbx]

Well, yes and no. GMT is GMT and does not change with DST or what they call
Summer Time. But the GMT+-0 timezone for London does change. It is
important to make the distiction that the GMT timezone for London is not
GMT.


No, it is correct. But remember, during Summer time, the time in Greenwich
or London is not GMT but actually GMT(+1). Vista is showing timezones, not
GMT. So when you select the timezone for Greenwich, it applies the DST or
Summer Time offset when adjusting the clock. Since Casablanca does not
switch to DST or Summer Time, that is why it's GMT is an hour different than
Greenwich's GMT.


That is because .NET is using the GMT time, not the timezone for Greenwich.
In other words, it doesn't worry about applying any DST offset to the base
since true GMT never changes in that manner. It does apply the DST offset
to your local timezone.


You've got it. The only thing I'll add is that what it is showing is the
GMT offset, not GMT. And since for London there is no offset it just shows
GMT rather than GMT-8 or similar. But that displayed offset doesn't include
any variance for seasonal clock changes. So for your current Pacific
timezone you're not really currently GMT-8 but more appropriately GMT-8(+1).
And London is currently GMT(+1). Casablanca is currently GMT. The display
doesn't change based on seasonal time changes because that is not how
timezones are defined, but it is something that has to be taken into
consideration when calculating the difference in time between two timezones.

Tom-

thanks, this all makes sense now as Microsoft is indeed presenting
"time zones", which vary based on DST.

the problem is with the labelling... they call the London time zone:

"(GMT) Greenwich Mean Time: Dublin, Edinburgh, Lisbon, London"

perhaps they should just use the offset notation...

e.g.,

"GMT (+0:00): Dublin, Edinburgh, etc."

and wouldn't it be nice, then, if there was a way to display plain 'ol
simple UTC?