Incorrect Julian Day

[Rayven]

Given the following code:


QDate test( 2009, 2, 10 );

qDebug( ) << test.toJulianDay( ) << " " << QDate::fromJulianDay( 41 );


I would expect the results to be:
41 QDate( "Sat Feb 10 -4713" )

Instead I get the results
2452873 QDate( "Sun Feb 11 -4713" )

I know this result is not correct and I have submitted a bug to the Trolls (or is it Nokias now?). Here is a quick workaround, if you know the Julian Day:



QDate test( 2009, 1, 1 );
test = test.addDays( julianDate - 1 );

[ChrisW67]

Why would you expect test.toJulianDay( ) to give you 41 when test is set to 10 Feb 2009?

[Lykurg]

Why would you expect test.toJulianDay( ) to give you 41 when test is set to 10 Feb 2009?

Yepp!


Given the following code:


QDate test( 2009, 2, 10 );
qDebug( ) << test.toJulianDay( ) << " " << QDate::fromJulianDay( 41 );

[...]
Instead I get the results
2452873 QDate( "Sun Feb 11 -4713" )

Really? I get 2454873 QDate("Sun Feb 11 -4713")


JD 41.00000 is
BCE 4713 February 11 12:00:00.0 UT Sunday

JD 2454873.00000 is
CE 2009 February 10 12:00:00.0 UT Tuesday

[Rayven]

Why would you expect test.toJulianDay( ) to give you 41 when test is set to 10 Feb 2009?
Because DOY 41 is Feb 10, not the 11th.

So did you get this error too Lykurg?

[ChrisW67]

The Julian Day Number is not the day-of-year that you seem to be expecting. It is the whole number of days since January 1, 4713 BC Greenwich noon (Julian proleptic calendar), which is why you get a large number, and not 41. It is useful for calculating intervals between dates without fussing over leap years etc.
http://en.wikipedia.org/wiki/Julian_day_number

For the day of the year you would want something like:
QDate test(2009, 2, 10);
qDebug() << test << test.dayOfYear();which gives:
QDate("Tue Feb 10 2009") 41 as you are expecting.

[Rayven]

Ahh, then my understanding of Julian Day is incorrect! Thanks so much, it makes more sense now.

[Abeshenkov]

Julian date is not really correct. If we look to define a, but at least the same wikipedia, the Julian date is from half a day. In this era a new date begins at midnight. Correspondingly, at the beginning of our day Julian date is not a whole!
This is not to say that the need of local time zones go to Greenwich's time. And it may happen that even in Greenwich yesterday or tomorrow already.