Webmaster Forum / HTML, CSS, Scripts / JavaScript / August 2005

JRS:  In article <df1vn6026dh@news2.newsguy.com>, dated Tue, 30 Aug 2005
11:58:36, seen in news:comp.lang.javascript, Jim Davis
<newsmonkey@vboston.com> posted :

Please do not start a new thread to continue an existing discussion.  As
a thread develops, it is taken for granted that some progress may be
made.

There are 60 minutes in each hour; I see no other possible difficulty.
In fact, you do not even need to know that.  Perhaps you should explain
your problem more clearly.

Remember that you should not use  new Date(Y, M, D, h, m, s, ms)   as
that muddles the situation with your local zone.  Use   new
Date(Date.UTC(,,,,,,)) .

Read the FAQ; see below.  Use extant resources before reinventing
wheels.

Consider
       new Date(Date.UTC(Y, M, D, h+oh, m+om, s, c))
where oh, om are the offsets and maybe + should be - ; that's not
explicit in the cited resource but should be obvious after reading it.

Consider whether the input should be validated; perhaps not, if your
data source is reliable.

Well, I took a nap and then was able to figure it out.

There are two general methods for doing this.  I'll be used the term
"source" in relation to the given time and "target" in relation to the local
time zone (so the problem becomes "how to convert a source timezone to a
target timezone):

1) A two stage conversion.  Step one uses the source timezone offset to
generate a UTC datetime.  We then use the target offset to generate the
local time.

This works just fine... but it's rather inelegant in my opinion (needing the
middle-man irks me).

2) Do the basic math (never a personal strong point).  Subtracting the
target from the source gives us our answer.  For example:

Source offset is +1:00, target is -4:00: (+1 - -4) = 5
Source offset is -5:00, target is -5:00: (-5 - -5) = 0
Source offset is -7:00, target is 2:00: (-7 - +2) = -9

And so forth.  The resulting number is the offset between the source date
and the target date: modify the target by the result and the conversion is
complete.

I hope this helps somebody else.  ;^)

Jim Davis

The sample below converts a timestamp to local time.  If a zone is
present, it is used to create a local time based on the offset.

Each component must be a proper 'bit' of the datetime in the format
you've specified, i.e. the date must be yyyy-m-d, the time must be
Thh:mm:ss and the zone must be +/-hh:mm.  It will tolerate not having
leading zeros.

How you handle not having some parts of the date can done elsewhere,
e.g. if you get '2005' you can decide if you want to make it 2005-01-01
or 2005-currentMonth-currentDate, it's up to you.  Same with the time.
If no timezone offset is specified, it will be the local one.

Decimal seconds in the input time are removed.

The input date separator can be '-' or '/' as hyphens are converted to
slashes.

<form action="">
<input type="text" name="time" value="1997-07-16T19:20:30.45+01:00"
size="50">
<input type="button" value="Convert" onclick="

  var x, tz, tStr = this.form.time.value;

  // Get timezone
  tz  = tStr.match(/[\+\-]\d{1,2}:\d{1,2}$/);

  // Remove timezone
  tStr = tStr.replace( tz, '');

  // Split date and time
  tStr = tStr.split('T');

  // Format date string
  x = tStr[0].replace(/-/g,'/');

  // If there's a time
  // Remove decimal seconds and append it to the date string
  if ( tStr[1] ) x += ' ' + tStr[1].match(/^[^\.]+/);

  // Add timezone offset if there is one
  if ( tz ) x += ' ' + tz;

  // Generate a date object using the string
  var localTime = new Date( x );

  // Show the world what we've done
  var msg = document.getElementById('zz')
  msg.innerHTML  = 'Datetime string: ' + x;
  msg.innerHTML += '<br>Timezone: ' + (tz || '<i>No timezone</i>');
  msg.innerHTML += '<br>Local time: ' + localTime.toString();

">
<br>
<input type="reset">
</form>
<p id="zz"></p>