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2004-04-20 12:04:06 AM
delphi165
I wasn't sure where to post this so I posted it here.
How do I take a UTC date FROM THE PAST and convert it into a local date?
GetLocalTime() seems to only deal with the current date/time.
 

"Richard A Stevens" <XXXX@XXXXX.COM>wrote
(1) How is your input formatted or what type
of variable does it use?
(2) What is the most historic date that you are
interested in?
(3) What do you mean by "local date" and what type
of variable should the output use?
Regards, JohnH
 

(1) Currently dates are in a TDateTime variable.
(2) Two years in the past.
(3) Converting past UTC times into a local timezone time including daylight
savings bias. i.e. UTC time to Pacific Standard Time or Pacific Daylight
Time, whatever the case may be.
_____________________
"John Herbster" <herb-sci1_AT_sbcglobal.net>writes

Richard A Stevens in <4083f7ed$XXXX@XXXXX.COM>writes:
and it is been working so far. it is version 1 (it works, who cares how
ugly it is?), so I am sure some here will probably attack the code. But,
the bottom line is, it is been working for me. Delphi 7 code.
const
RSSDateFormat = 'ddd, dd mmm yyyy hh:nn:ss ''GMT''';
function GetTimeZoneBias: Integer;
var
TimeZoneInfo: TTimeZoneInformation;
begin
case GetTimeZoneInformation(TimeZoneInfo) of
TIME_ZONE_ID_STANDARD: Result := TimeZoneInfo.Bias +
TimeZoneInfo.StandardBias;
TIME_ZONE_ID_DAYLIGHT: Result := TimeZoneInfo.Bias +
TimeZoneInfo.DaylightBias;
else
Result := 0;
end;
end;
function GetUTC: TDateTime;
var
stim: SYSTEMTIME;
begin
GetSystemTime(stim);
result:=SystemTimeToDateTime(stim);
end;
function GetUTCDelta: TDateTime;
begin
Result := Now - GetUTC;
end;
function DateTimeToRSSTime(Value: TDateTime; ApplyLocalBias: Boolean =
True): WideString;
var
Bias: Integer;
begin
Bias := GetTimeZoneBias;
if (Bias <>0) and ApplyLocalBias then
Value := Value - GetUTCDelta;
Result := FormatDateTime(RSSDateFormat, Value); { Do not localize }
end;
function RSSTimeToDateTime(Value: string; ApplyLocalBias: Boolean =
True): TDateTime;
var
Bias: Integer;
y, m, d, h, min, s: word;
begin
// ddd, dd mmm yyyy hh:nn:ss GMT
// 1234567890123456789012345
y := StrToInt(Copy(Value, 13, 4));
m := Pos(LowerCase(Copy(Value, 9, 3)),
'jan feb mar apr may jun jul aug sep oct nov dec') // do not
localize
div 4 + 1;
d := StrToInt(Copy(Value, 6, 2));
h := StrToInt(Copy(Value, 18, 2));
min := StrToInt(Copy(Value, 21, 2));
s := StrToInt(Copy(Value, 24, 2));
Result := EncodeDateTime(y, m, d, h, min, s, 0);
Bias := GetTimeZoneBias;
if (Bias <>0) and ApplyLocalBias then
Result := Result + GetUTCDelta;
end;
--
John Kaster, Borland Developer Relations, bdn.borland.com
Add a feature/Fix a bug: qc.borland.com
Get source: cc.borland.com
 

"Richard A Stevens" <XXXX@XXXXX.COM>wrote
"UTC" times to be what the wall clock was showing back
then and there. (Please correct me if I am wrong!)
I suspect that the Windows SDK or someone has a neat
routine for converting historical "UTC" times to historical
wall calendar and clock dates and times based on a
specific TIME_ZONE_INFORMATION record that is
assumed to have applied in the past. While I have not
done exactly this yet, you should be able to get and
interpret the fields in the TIME_ZONE_INFORMATION
record and apply the offset to your historical "UTC" time
to make the wall time.
Regards, JohnH
 

John:
That's exactly what I want to do. Daylight Bias, as I understand it, only
deals with the current system time, not whether a date in the past had any
Daylight Bias.
Rich
"John Herbster" <herb-sci1_AT_sbcglobal.net>writes

Richard A Stevens writes:
or just made them up myself ;-)
In the first one, the FileTimeToLocalTime is the real call, the others
are just for getting there from TDateTime and getting back to TDateTime.
In the second one, you need to pass info about the timezone, but it
only works on NT.
O, and one thing to note: it show the time in relation to the current
time. So a time in the past, for example before the last daylightsaving
time started will show off by one...: So when it was in fact 8:00 in
februari it will now show as 9:00 (just as NT/XP is showing times from
files in the past).
function UTCToLocalTime(aUTCTime: TDateTime; aDontConvertIfTimeIsNull:
Boolean=False): TDateTime;
var
LT, UT: TSystemTime;
FileTime, LocalFileTime: TFileTime;
begin
DateTimeToSystemTime(aUTCTime, UT);
if aDontConvertIfTimeIsNull and (UT.wHour=0) and (UT.wMinute=0) and
(UT.wSecond=0) and (UT.wMilliseconds=0) then
begin
Result := aUTCTime;
Exit;
end;
if not SystemTimeToFileTime(UT, FileTime) then
RaiseLastOSError;
if not FileTimeToLocalFileTime(FileTime, LocalFileTime) then
RaiseLastOSError;
if not FileTimeToSystemTime(LocalFileTime, LT) then
RaiseLastOSError;
Result := SystemTimeToDateTime(LT);
end;
function UTCToLocalTime(aUTCTime: TDateTime; lpTimeZoneInformation:
PTimeZoneInformation): TDateTime;
var
LT, UT: TSystemTime;
begin
// function works only on NT!
DateTimeToSystemTime(aUTCTime, UT);
if not SystemTimeToTzSpecificLocalTime(lpTimeZoneInformation, UT, LT)
then
RaiseLastOSError;
Result := SystemTimeToDateTime(LT);
end;
Regards,
Tjipke van der Plaats
--
Update your version information in your project from the command line?
Use ChangeRes! See www.tiriss.com/changeres
 

John Kaster (Borland) writes:
.......
John, this won't work, because your "GetTimeZoneBias"
and GetUTCDelta functions give the *current* timezone/
DST bias, not that of the two years old tdatetime that
Richard is trying to convert. One needs to determine
the bias for the specific date!
--
Arthur Hoornweg
(please remove the ".net" from my e-mail address)
 

JRS: In article <4084080b$XXXX@XXXXX.COM>, seen in news:borla
nd.public.delphi.non-technical, Richard A Stevens <XXXX@XXXXX.COM>
posted at Mon, 19 Apr 2004 10:12:51 :
Windows will give the present Summer Time rules for the current
location, but that is no great help. You need the past rules for the
desired local location.
You do not say whether the current location will always be PST/PDT, or
whether it will always be USA, or whether it may be anywhere; that
matters. The US rules are well-defined, but odd parts of the US have no
Summer Time. The CA ones are non-identical.
The algorithm is to take the UTC year, consider it as the destination
year (It will be, except for southern hemisphere destinations for a
fraction of a day, which does not ultimately matter), then calculate the
local transition date/times for that year by local rules, then convert
those to UTC, and compare. Now you know whether it is summer or winter
at the UTC in question. Adjust by the local winter offset, and if
needed by the clock change (which is one hour almost everywhere).
My <URL:www.merlyn.demon.co.uk/uksumtim.htm>discusses Summer
Time, for the UK, the EU, and other places that might be of interest;
<URL:www.merlyn.demon.co.uk/js-date3.htm>and
<URL:www.merlyn.demon.co.uk/js-date5.htm>have code in
javascript. <URL:www.merlyn.demon.co.uk/programs/dateprox.pas>
has Pascal for calculating the EU change dates, adaptable no doubt for
NA.
Indeed, js-date5.htm#Demo will do the job requested (so can be used as a
check) if you select NY then change EST5EDT to PST8PDT.
--
?John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 MIME. ?
Web <URL:www.merlyn.demon.co.uk/>- w. FAQish topics, links, acronyms
PAS EXE etc : <URL:www.merlyn.demon.co.uk/programs/>- see 00index.htm
Dates - miscdate.htm moredate.htm js-dates.htm pas-time.htm critdate.htm etc.
 

In article <4083f7ed$XXXX@XXXXX.COM>, XXXX@XXXXX.COM
says...
( www.esbconsult.com.au/ ) has a swack of routines for futzing
around with dates.
--
John
Life is complex. It has real and imaginary parts
 

Thanks Arthur. I see you understand what I am trying to do..
Richard Stevens
_____________________
"Arthur Hoornweg" <XXXX@XXXXX.COM>writes

Arthur Hoornweg in <4084c3d2$XXXX@XXXXX.COM>writes:
attention.
--
John Kaster, Borland Developer Relations, bdn.borland.com
Add a feature/Fix a bug: qc.borland.com
Get source: cc.borland.com
 

In addition to the algorithms that Dr John and others have
proposed, you might find this Google search interesting
www.google.com/search
There may be some Internet accessible tables and some
programming components available for helping with the
historical corrections.
Regards, JohnH
 

"Dr John Stockton" <XXXX@XXXXX.COM>writes
<snip>
Prior to 1883, everyone used mean solar time (no time zones at all) and
daylight saving time didn't even begin until WWI.
The portion of Indiana which is in the Eastern Time Zone refrains from DST,
but certain counties on the border with neighboring states use it. IIRC,
the entire US stayed on it for two years during the Nixon administration,
due to the oil shortage. I think that Arizona does not observe it, but
oddly the Native Americans do.
See:
webexhibits.org/daylightsaving/g.html
Calculating leap years is trivial compared to this. Did you know that
Windows 3.1 could never become Y2K compliant because the MS programmers just
assumed that 2000 is not a leap year? Most centuries are not, but any
divisible by 4 are. They thought they knew the details, but didn't look
them up.
Kirk Halgren
"One word sums up probably the responsibility of any Governor, and that one
word is 'to be prepared'." -- George W. Bush
 

Kirk Halgren writes:
--
Wayne Niddery - Logic Fundamentals, Inc. (www.logicfundamentals.com)
RADBooks: www.logicfundamentals.com/RADBooks.html
"True peace is not the absence of tension, but the presence of
justice." - Martin Luther King, Jr.