Recently I needed a way of concerting back and forth ISO 8601 DateTime values used in XML from Delphi.

Thoug the Delphi DateUtils unit has some ISO 8601 features for calculating week numbers, you actually need to the XSBuiltIns unit for converting back and forth to ISO 8601 text representation of a DateTime.

I remembered answering the related In Delphi is there a function to convert XML date and time to TDateTime question on StackOverflow a while ago (and forgot that this was back in 2009 <g>).

ISO 8601 dates can either include a time-zone, or be in UTC, which is not part of that answer. So lets elaborate on that answer a bit now:

UTC times in ISO 8601 format end in a Z time zone designator like this:

    <Created>2011-06-29T17:01:45.505Z</Created>

The Z UTC indicator is basically a shortcut for a timezone offset of +00:00 or -00:00, effectively being a zero (or zulu) timezone.

Nonzero timezones start with an optional + or -, followed by the hours and minutes offset from UTC, for instance +01:00 for the Central European Time zone.

    <Created>2011-06-29T17:01:45.505+01:00</Created>

When you want historical time zones, then you need the Delphi TZDB interface to the historical TZ database.

To do the timezone calculations, I used the TimeZoneBias function from Indy, which is either in the IdGlobal unit (Indy <= version 9) or the IdGlobalProtocols unit (Indy 10 and up).

Conversion is done by using the TXSDateTime (that like all the XS conversion classes descends from TRemotableXS in the InvokeRegistry unit).

Most of the classes descending from TRemotableXS contain two methods: NativeToXS and XSToNative doing the underlying conversions.

Since I didn’t need the historical reference in the TZDB, this is the code that I came up with:

unit Iso8601Unit;

interface

type
  TIso8601 = class(TObject)
  public
    class function DateTimeFromIso8601(const Value: string): TDateTime; static;
    class function UtcDateTimeToIso8601(const Value: TDateTime): string; static;
    class function DateTimeToIso8601(const Value: TDateTime): string; static;
    class function UtcNow: TDateTime; static;
    class function ToUtc(const Value: TDateTime): TDateTime; static;
    class function FromUtc(const Value: TDateTime): TDateTime; static;
  end;

implementation

uses
  IdGlobalProtocols, {IdGlobal for Index   SysUtils,
  XSBuiltIns;

class function TIso8601.DateTimeFromIso8601(const Value: string): TDateTime;
begin
  with TXSDateTime.Create() do
  try
    XSToNative(value); // convert from WideString
    Result := AsDateTime; // convert to TDateTime  finally
  finally
    Free();
  end;
end;

class function TIso8601.UtcDateTimeToIso8601(const Value: TDateTime): string;
begin
  with TXSDateTime.Create() do
  try
    AsUTCDateTime := Value;
    Result := NativeToXS; // convert to WideString
  finally
    Free();
  end;
end;

class function TIso8601.DateTimeToIso8601(const Value: TDateTime): string;
begin
  with TXSDateTime.Create() do
  try
    AsDateTime := Value; // convert from TDateTime
    Result := NativeToXS; // convert to WideString
  finally
    Free();
  end;
end;

class function TIso8601.UtcNow: TDateTime;
begin
  Result := ToUtc(Now);
end;

class function TIso8601.ToUtc(const Value: TDateTime): TDateTime;
var
  Bias: TDateTime;
begin
  Bias := TimeZoneBias;
  Result := Value + TimeZoneBias;
end;

class function TIso8601.FromUtc(const Value: TDateTime): TDateTime;
var
  Bias: TDateTime;
begin
  Bias := TimeZoneBias;
  Result := Value - TimeZoneBias;
end;

end.

–jeroen

via In Delphi is there a function to convert XML date and time to TDateTime – Stack Overflow.