[fpc-devel] New functions for time zone

[LacaK]

Michael Van Canneyt  wrote / napísal(a):
>
>
> On Wed, 11 Jul 2012, LacaK wrote:
>
>> Hi *,
>> I noticed, that there was just added new functions for supporting TZ 
>> in rev. 21865 and 21866
>> Now function GetLocalTimeOffset: Integer returns offset *in minutes* 
>> and later in functions LocalTimeToUniversal and UniversalTimeToLocal 
>> are used commands to transform it to TDateTime using EncodeTime.
>> (f.e. EncodeTime(TZOffset div 60, TZOffset mod 60, 0, 0))
>>
>> My idea is change function GetLocalTimeOffset: TDateTime to return 
>> TDateTime (offset in fraction of days - native FPC datetime format)
>> i.e. current implementation + "Result := Result / MinsPerDay"
>> I think, that it will be useful, because in real applications we will 
>> need on each usage transform minutes to use it in Datetime calculations.
>> (LocalDateTime := UTCDateTime - GetLocalTimeOffset;)
>>
>> Then functions LocalTimeToUniversal and UniversalTimeToLocal will be 
>> simple addition or subtraction like:
>> function UniversalTimeToLocal(UT: TDateTime; TZOffset : TDateTime): 
>> TDateTime;
>> begin
>>  Result := UT - TZOffset;
>> end;
>>
>> What do you think? It will simplify all.
>
> There are 2 sides to this medal. The function returns a number of 
> timezones;
> This is usually expressed as a number of hours and minutes (the OS 
> returns it so). The result is in 'natural' units.
>
AFAIK f.e. GetTimeZoneInformation under Windows return bias in minutes

> Returning it as a timestamp - while useful by itself - seems rather odd.
>
May be, that I expressed it wrong


My proposal is like:

Windows:
     function GetLocalTimeOffset: double;   <--
591            
592            var
593              TZInfo: TTimeZoneInformation;
594            
595            begin
596               case GetTimeZoneInformation(TZInfo) of
597                 TIME_ZONE_ID_UNKNOWN:
598                   Result := TZInfo.Bias;
599                 TIME_ZONE_ID_STANDARD:
600                   Result := TZInfo.Bias + TZInfo.StandardBias;
601                 TIME_ZONE_ID_DAYLIGHT:
602                   Result := TZInfo.Bias + TZInfo.DaylightBias;
603                 else
604                   Result := 0;
605               end;
+                 Result := Result / MinsPerDay;
606            end;

Unix:
                function GetLocalTimeOffset: double;   <--
1405            
1406            begin
+                  Result := -Tzseconds / SecsPerDay;
1408            end;


dateutil.inc:
function UniversalTimeToLocal(UT: TDateTime; TZOffset : double): TDateTime;
2467            
2468            begin
                    Result := UT - TZOffset;
2476            end;

Function LocalTimeToUniversal(LT: TDateTime;TZOffset: double): TDateTime;
2485            
2486            begin
                   Result := LT + TZOffset;
2494            end;

if TZOffset will be "day fraction" then we can simplyfy above mentioned 
functions, or am I wrong ?
Thanks
-Laco.