Converting the time from one timezone to another

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Nathan Sokalski - 18 Feb 2006 07:19 GMT

I asked a question about a week ago about how to get my local time, since my
application is running on a server halfway around the world. I have
determined that my code would look something like this:

Dim servertime As Date = Date.Now

Dim utctime As Date = servertime.ToUniversalTime()

Dim localtime As Date

I know that the last step would be to adjust the utctime value by the
appropriate amount using code such as utctime.AddHours(-5), but I am looking
for a way to get this value by supplying the timezone rather than an offset
(in other words, I am looking for a function that returns either a
System.TimeSpan or Integer when I enter the timezone) so that I can do
something such as

utctime.AddHours(GetTZOffset(TimeZones.EST))

Is there a function that does this, or any way to get the offset by
submitting the timezone? Thanks.

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Nathan Sokalski
njsokalski@hotmail.com
http://www.nathansokalski.com/

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William Stacey [MVP] - 18 Feb 2006 07:31 GMT

See my UTC project for sql. It has the c# class you need.
http://channel9.msdn.com/ShowPost.aspx?PostID=142586

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William Stacey [MVP]
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aweska don - 18 Feb 2006 08:18 GMT

Thanks

http://www.newbestweb.com owner

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Nathan Sokalski - 18 Feb 2006 17:53 GMT

I could not find any code or links to download code on the page you gave a
link to. Also, even though I will be using the code for SQL in some cases,
my goal is to generate a System.DateTime object that is the local time. If I
am missing the code, please let me know exactly where on your page it is.
Thanks.

Signature

Nathan Sokalski
njsokalski@hotmail.com
http://www.nathansokalski.com/

> See my UTC project for sql. It has the c# class you need.
> http://channel9.msdn.com/ShowPost.aspx?PostID=142586

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William Stacey [MVP] - 18 Feb 2006 23:44 GMT

There is a "Save" link at the bottom of the article. Easy to miss. The
APIs get and return DateTime, so you should be ok. Can use with sql or
without. Let me know if you still have problems.

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William Stacey [MVP]

|I could not find any code or links to download code on the page you gave a
| link to. Also, even though I will be using the code for SQL in some cases,
[quoted text clipped - 3 lines]
| > See my UTC project for sql. It has the c# class you need.
| > http://channel9.msdn.com/ShowPost.aspx?PostID=142586

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Stephany Young - 18 Feb 2006 21:01 GMT

Given the other newsgroups you cross-posted to, I have to assume that you
are talking about an ASP.NET application running on a web server halfway
around the world, and that your reference to 'local' actually refers to the
machine upon which you are viewing the generated 'page' in a web browser.

Working on that principle, I also assume that you want to either:

1. Carry out some calculation at the server using a date/time pasesed
from the client that represents the a point in time at the client
expressed in terms of the time zone where the server is physically
situated,

or

2. Carry out some calculation at the client using a date/time passed
from the server that represents a point of time at the server
expressed in terms of the time zone where the client is physically
situated.

Either way, the most critical aspects of the whole exercise are that:

1. The time zone for both the 'server' and 'client' machines are
set correctly,

and

2. The date and time for both the 'server' and 'client' machines are
regularly corrected using a reliable source.

The key to solving the 'problem' is to forget about time zones per se, and
to think in terms of a common point of comparison and the differences from
that point.

In terms of date/time we, happily, have the concept of Universal Time
Coordinated (UTC) which provides a common reference point for expressing
date/time anywhere in the world. For those who haven't caught up yet, UTC
used to be known as Greenwhich Mean Time (GMT).

We know that your 'client' machine is physically situated somewhere within
the Eastern Time (US & Canada) time zone which is 5 hours behing UTC. We
will take you literally and assume that your 'server' machine is physically
located 'halfway around the world', in a time zone that is 5 hours ahead of
UTC.

As long as both machines are configured correctly, they will be aware of
daylight saving factors relating to their respective time zones and,
therefore daylight saving adjustments will be applied to any date/time
calculations automatically.

For 'one end' to be able to express date/time factors in terms of the 'other
end' then one end has to:

1. Know in advance where the other end is, in relation to UTC,

or

2. Be told as required what the date/time is at the other end, in terms of
UTC.

Knowing in advance where the other end is, in relation to UTC, is not really
practical for an ASP.NET application because it would need to know in
advance the location of every machine that could ever submit a request ot
it.

So that leaves us with communicating the necessary information as and when
required.

The DateTime structure nicely presents us with properties and methods for
dealing with values expressed in terms of UTC.

To 'capture' the local date and time as UTC we use:

Dim _utc As DateTime = DateTime.UtcNow

MySendUTCToOtherEnd(_utc)

To convert a UTC date and time to local we use:

Dim _remoteutc as DateTime = MyGetUTCFromOtherEnd()

Dim _local As DateTime = _remoteutc.ToLocalTime

_local now contains a value that represents the date and time from the
remote machine expressed in terms of the time zone where the local machine
is physically located.

To illustrate with an example:

Client machine is UTC - 5 hours
Server machine is UTC + 5 hours

Client local date/time = February 18, 7:00 PM
Server local date/time = February 19, 5:00 AM

UTC value passed from client = February 19, 0:00 AM

_local as calculated by server = February 19, 5:00 AM

and, in reverse:

Client machine is UTC - 5 hours
Server machine is UTC + 5 hours

Server local date/time = February 19, 5:00 AM
Client local date/time = February 18, 7:00 PM

UTC value passed from server = February 19, 0:00 AM

_local as calculated by client = February 18, 7:00 PM

>I asked a question about a week ago about how to get my local time, since
>my application is running on a server halfway around the world. I have
[quoted text clipped - 17 lines]
> Is there a function that does this, or any way to get the offset by
> submitting the timezone? Thanks.

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Nathan Sokalski - 18 Feb 2006 22:31 GMT

Did you read my posting? I understand the concept of UTC, and the process of
getting from servertime to localtime (I even showed the code for this in my
posting). My question is "Does .NET have a way of telling me how many hours
to subtract, perhaps a function that returns the number of hours to
add/subtract from UTC that might be something like this:

UtcOffsetForTimezone("EST") would return -5

If I wanted to get the local time for Eastern Standard Time, I could very
easily just use

Date.UtcNow.AddHours(-5)

The reason I don't like this method is because since most people know what
timezone they live in by it's name rather than it's UTC offset, I would like
a way to enter the timezone as a parameter in order to find the offset.
Sure, I could lookup all the timezones and their offsets myself, but I would
expect this to already be a part of ASP.NET. This may sound lazy, but if you
look at all the other things that are easy but ASP.NET have done for you,
it's not an unreasonable thing to ask.

Signature

Nathan Sokalski
njsokalski@hotmail.com
http://www.nathansokalski.com/

> Given the other newsgroups you cross-posted to, I have to assume that you
> are talking about an ASP.NET application running on a web server halfway
[quoted text clipped - 128 lines]
>> Is there a function that does this, or any way to get the offset by
>> submitting the timezone? Thanks.

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Stephany Young - 18 Feb 2006 23:07 GMT

The point I was making was that you don't have to worry about time zones at
all.

Just have your client side code calculate the utc offset and pass the result
to the server.

That said, the answer to your question is No, .NET does not have an FCL
equivalent method such as UtcOffsetForTimezone("EST").

> Did you read my posting? I understand the concept of UTC, and the process
> of getting from servertime to localtime (I even showed the code for this
[quoted text clipped - 148 lines]
>>> Is there a function that does this, or any way to get the offset by
>>> submitting the timezone? Thanks.

Reply to this Message

William Stacey [MVP] - 18 Feb 2006 23:50 GMT

Did you even look at the code I linked for you. It does exactly what you
want. I agree that this sort of thing should be in the BCL, but for now,
you need to roll it yourself. Do you have any issues with the code?

Signature

William Stacey [MVP]

| Did you read my posting? I understand the concept of UTC, and the process of
| getting from servertime to localtime (I even showed the code for this in my
[quoted text clipped - 148 lines]
| >> Is there a function that does this, or any way to get the offset by
| >> submitting the timezone? Thanks.

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Sandor Heese - 23 Feb 2006 19:25 GMT

have a look at this MSDN article:

Coding Best Practices Using DateTime in the .NET Framework
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/d
atetimecode.asp

it contains a lot of info about how to convert between timezones etc.

Sandor

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clintonG - 02 Mar 2006 16:13 GMT

Thanks so much for posting that URL Sandor...

<%= Clinton Gallagher

> have a look at this MSDN article:
>
[quoted text clipped - 26 lines]
>> Is there a function that does this, or any way to get the offset by
>> submitting the timezone? Thanks.

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Converting the time from one timezone to another

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