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Date is broken for years below one. [message #26542]

Mon, 10 May 2010 16:06

Novo
Messages: 1358
Registered: December 2006

Ultimate Contributor

const Date date1(0, 1, 1);
ASSERT(date1.IsValid());
const int n = date1.Get();

Date date2;
date2.Set(n);

ASSERT(date1 == date2);

I couldn't find description of an algorithm used in UPP, but other libraries (like BOOST) use Julian Day Number to convert date to a number of days. Algorithm for calculation seems to be much simpler in this case.

http://beta.boost.org/doc/libs/1_42_0/boost/date_time/gregor ian_calendar.ipp

http://www.faqs.org/faqs/calendars/faq/part2/

http://en.wikipedia.org/wiki/Julian_day

Regards,
Novo

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Re: Date is broken for years below one. [message #26544 is a reply to message #26542]

Mon, 10 May 2010 18:15

dolik.rce
Messages: 1789
Registered: August 2008
Location: Czech Republic

Ultimate Contributor

Hi Novo,

Quite similar topic was discussed here few months ago, see this thread.
As someone noted

Quote:

not many dates are known so detailed from the years before 1582

I don't think this is a serious problem. Most of the algorithms using date should work just fine.

If you really need to count days, you can try to fix Date::Get() and Date::Set(). They contain just plain arithmetics, but it gets tricky when year<=0 Wink

It is because of how the integral division works. For starter, I *think* that Date::Get)) should be

int Date::Get() const {
	return year * 365 + s_month_off[month - 1] + (day - 1) +
	       (year - 1) / 4 - (year - 1) / 100 + (year - 1) / 400 + (year>0) +
	       (month > 2) * IsLeapYear(year);
}

For Date::Set() it is bit more complicated, there you would have to add similar correction and additionally also add few more if blocks, because now all the ifs for counting leap years check just for (d>=...).

Best regards,
Honza

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