Why is Excel serial date-time to NSDate conversion two days, one hour and 40 minutes ahead? - objective-c

Question

I'm converting dates from an Excel spreadsheet to NSDate's, but for some reason they always come out two days ahead: Sundays come out as Tuesdays, etc.
My conversion method is based on the following info from cpearson.com:
Excel stores dates and times as a number representing the number of
days since 1900-Jan-0, plus a fractional portion of a 24 hour day:
ddddd.tttttt . This is called a serial date, or serial date-time.
(...) The integer portion of the number, ddddd, represents the number
of days since 1900-Jan-0. (...) The fractional portion of the number,
ttttt, represents the fractional portion of a 24 hour day. For
example, 6:00 AM is stored as 0.25, or 25% of a 24 hour day.
Similarly, 6PM is stored at 0.75, or 75% percent of a 24 hour day.
- (NSDate *)dateFromExcelSerialDate:(double)serialdate
{
if (serialdate == 0)
return nil;
NSTimeInterval theTimeInterval;
NSInteger numberOfSecondsInOneDay = 86400;
double integral;
double fractional = modf(serialdate, &integral);
NSLog(#"%# %# \r serialdate = %f, integral = %f, fractional = %f",
[self class], NSStringFromSelector(_cmd),
serialdate, integral, fractional);
theTimeInterval = integral * numberOfSecondsInOneDay; //number of days
if (fractional > 0) {
theTimeInterval += numberOfSecondsInOneDay / fractional; //portion of one day
}
NSCalendar *nl_gregorianCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSTimeZone *nl_timezone = [[NSTimeZone alloc] initWithName:#"Europe/Amsterdam"];
[nl_gregorianCalendar setTimeZone:nl_timezone];
NSDateComponents *excelBaseDateComps = [[NSDateComponents alloc] init];
[excelBaseDateComps setMonth:1];
[excelBaseDateComps setDay:1];
[excelBaseDateComps setHour:00];
[excelBaseDateComps setMinute:00];
[excelBaseDateComps setTimeZone:nl_timezone];
[excelBaseDateComps setYear:1900];
NSDate *excelBaseDate = [nl_gregorianCalendar dateFromComponents:excelBaseDateComps];
NSDate *inputDate = [NSDate dateWithTimeInterval:theTimeInterval sinceDate:excelBaseDate];
NSLog(#"%# %# \r serialdate %f, theTimeInterval = %f \r inputDate = %#",
[self class], NSStringFromSelector(_cmd),
serialdate, theTimeInterval,
[self.nl_dateFormatter stringFromDate:inputDate]);
return inputDate;
}
The spreadsheet was produced in the Netherlands, presumably on a Dutch version of Microsoft Excel.
Spreadsheet date Sunday July 6, 2014 00:00 yields the following results:
dateFromExcelSerialDate:
serialdate = 41826.000000, integral = 41826.000000, fractional =
0.000000 theTimeInterval = 3613766400.000000 inputDate = 08 jul. 2014 01:40
Similarly, Sunday July 13, 2014 00:00 yields:
serialdate = 41833.000000, integral = 41833.000000, fractional =
0.000000 theTimeInterval = 3614371200.000000 inputDate = 15 jul. 2014 01:40
I can correct the output by subtracting 2 days, one hour and 40 minutes:
theTimeInterval -= ((60 * 60 * 24 * 2) + (60*60) + (60*40));
but I have no idea how robust that is.
That difference of two days made me think it had something to do with leap year corrections, so I tried to let the calendar do the calculations by adding the NSTimeInterval seconds to the excelBaseDate, like so:
NSDateComponents *comps = [[NSDateComponents alloc] init];
[comps setSecond:theInterval];
NSDate *inputDate = [nl_gregorianCalendar dateByAddingComponents:comps
toDate:excelBaseDate
options:0];
Strangely enough, that gave me dates somewhere in the 1870's. Who knows what is going on?

Answer 1

there are two things here:
your start date is 1900-Jan-1 but your referred description clearly says: the reference is 1900-Jan-0 – you may add an extra day here;
year 1900 was not a leap-year – you may add an extra day here;
I guess, this is pretty much the reason why you get two extra days every occasion.
Microsoft knows about that, see more about the topic here.