Thread: Help on converting date

Convert the original format into a unix timestamp, ie seconds since 1970, Then use the C standard time.h functions to convert it into the new format.

ie

Code:

SECONDSINAYEAR=365.25 * (24 * pow(60, 2))
SECONDSINAWEEK=7 * (24 * pow(60, 2))

year = 2000
ww = 24
d = 5

yearOffset = year - 1970
// etc for weeks / days

secondsSinceEpoch = (yearsOffset * SECONDSINAYEAR) + (weekOffset * SECONDSINAWEEK)

Or something

How do i converet 7403(2007, week number 40, wednesday) to a unix time?

Edit:
Some years start with week number 53

Or to put it another way:
How do i get the week number of 01. january of a year?

Here's a good start, Note it uses the leap year approximation method see http://en.wikipedia.org/wiki/Leap_year

Code:

time_t makeTimestamp(int year, int week, int day)
{
    time_t secs = 0;
    int daysPerYear = 365;
    
    /* if it's a leap year give it 366 days */
    if(isLeapYear(year) != 0)
        daysPerYear = 366;
    
    /* add the years in seconds from 1970 */
    secs += (year - 1970) * (daysPerYear * (24 * (60 * 60)));
    
    /* add the weeks, etc */
    
    return secs;
}

/* returns non-zero if the year is a leap year */
int isLeapYear(int year)
{
    if(year % 400 == 0 || year % 4 == 0)
        return 0;       /* leap */
    else if(year % 100 == 0)
        return 1;       /* not a leap */
    return 2;
}

All you have to do is make it pretty, add weeks and days then do whatever with the timestamp.

If the week is 47 then it'll be,

Code:

weeksInDays = 47 * 7
daysSinceStartOfYear = weeksInDays - (7 - dayOfWeek)

Or something, I don't know -- I don't like time

OK, so the first question is "what is the definition of a workweek". You need to know which week is "week 1" and at what date that week starts, since, as you say, some years end with a week 53 that overlaps into the next year.

Once you know which date week one starts, you then add the number of days that your 40 weeks (40 * 7 = 280 days), combined with the day within the week.

Add the number of days to the "start of week 1 date", and you've got a date.

Since UNIX time is "seconds since 1 Jan 1970", you multiply the number of days by 24 * 60 * 60 and you get the number of seconds into the year.

--
Mats

Well I was sortof on track

Originally Posted by knutso

So is there a function like: weeknumber('2007.01.01');?

I'm not aware of a "commonly available" one like that. It's not helped much by different countries having different definitions of "week number".

I wrote something to solve this a long time ago, which works out the day of December 31st of previous year, (because if it's before wednesday, the first week is week 53, otherwise week 1), then calculates the week number by calculating number of days between week 1 and the "wanted" date. This was quite a while ago, and I had a quick look to see if I could find it, but I wasn't able to find it right now... [It would be in Pascal anyways, so you would have to convert the algorithm to C].

Edit: Found it!

Code:

FUNCTION WeekNumber{date: String[h]): WORD};

  VAR
    day,
    month,
    year:        WORD;
    firstOf:     INTEGER;
    dayNum:      INTEGER;
    dayIntoYear: INTEGER;
    firstMonday: INTEGER;

  FUNCTION FirstMondayInYear(y: WORD): INTEGER;

    (* This calculates the Monday of Week 1 of the year we are
       interested in, which actually may be the in the previous
       year, since that week may begin in December. *)

    VAR
      x:    INTEGER;
      d:    WORD;
      step: INTEGER;

    BEGIN
      x := CalcDayNum(y, 1, 1);
      d := WeekDayNum(x);
      IF d < Thursday THEN
        step := -1
      ELSE
        step := 1;
      WHILE d <> Monday DO BEGIN
        x := x + step;
        d := WeekDayNum(x);
      END;
      FirstMondayInYear := x;
    END;


  FUNCTION CalcWeekNumber(year, month, day: WORD): WORD;

    BEGIN
      firstMonday := FirstMondayInYear(year);
      firstOf := CalcDayNum(year, 1, 1);
      dayNum := CalcDayNum(year, month, day);
      IF dayNum < firstMonday THEN
        CalcWeekNumber := CalcWeekNumber(year-1, 12, 31)
      ELSE BEGIN
        IF dayNum >= FirstMondayInYear(year+1) THEN
          CalcWeekNumber := CalcWeekNumber(year+1, 1, 1)
        ELSE
          CalcWeekNumber := (dayNum - firstMonday) DIV 7 + 1;
      END;
    END;


  BEGIN
    SplitDate(date, year, month, day);
    WeekNumber := CalcWeekNumber(year, month, day);
  END;

Edit2: That is missing a few crucial functions. The CalcDayNum() is basicily a integer representing the number of days from a set date (such as 1 Jan 1970). Likewise, we need a function to figure out what day of the week a certain date is - there's a post recently that does that, or you can use the standard C time functions to get the day of week from a "time_t" type.

--
Mats

I hope you've got a number of sample input dates and their correct outputs to use as test cases. This is something that would very much bennefit from having some unit tests, and it would be very easy to test also.

Make sure you pick some weird dates in that case, such as 29Feb on a leap year, or some other weird ones.

Originally Posted by knutso

The date format is from a lotnumber in a barcode, hence it has to be as short as possible.

In that case, I don't know why they didn't simply replace WWD with the 3-digit number for the day of the year since it's the same length.