[fpc-devel] Problem with Now() and time changed by ntpd

[Hans-Peter Diettrich]

Sven Barth schrieb:
> Am 03.11.2011 02:39, schrieb Hans-Peter Diettrich:
>> IMO we have to face a problem very similar to Ansi/UTF-8/16: A TDateTime
>> variable can contain local time in a number of timezones (Ansi), or UTC
>> values (UTF), which must be interpreted accordingly, e.g. in
>> DateTimeToStr().
>>
>> When Delphi compatible Now() provides local time TDateTime (including
>> DST, as is on Windows), and DateTimeToStr() etc. expect a local time
>> argument, a full emulation of these functions has to be provided on
>> other platforms as well.
> 
> But even in Delphi the programmer has to keep track whether a TDateTime 
> contains a local or UTC based time value (e.g. if he converted the local 
> time returned by Now to UTC).

Right, just like the user currently must remember whether an AnsiString 
contains Ansi or UTF-8. But how do you want to get the UTC and put it 
into a TDateTime?

> And functions like DateTimeToStr don't care whether a time value is 
> local or UTC and in my opinion they even MUST NOT.

Splitting the TDateTime into year, month etc. is done by a DecodeDate... 
function, that *assumes* that TDateTime contains a local time. When you 
feed it an UTC time, the result is unusable.

> If I use such a 
> function I want the value of the given TDateTime printed to a string no 
> matter what timezone I'm currently in.

A TDateTime only contains the number of days since the begin of the 
epoch (of the encoded date). Which epoch should apply?
The time part contains the fraction of the day, based on midnight of 
which timezone?

DoDi