Re: Converting the time from one timezone to another
- From: "Stephany Young" <noone@localhost>
- Date: Sun, 19 Feb 2006 10:01:34 +1300
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
"Nathan Sokalski" <njsokalski@xxxxxxxxxxx> wrote in message
news:OZBJ1uFNGHA.2300@xxxxxxxxxxxxxxxxxxxxxxx
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.
--
Nathan Sokalski
njsokalski@xxxxxxxxxxx
http://www.nathansokalski.com/
.
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