How to Get Calendar Week from Date.

[awolff01]

I need to determine what week of the year it is.

For example..01/01/2004 is in week 1
01/31/2004 is in week 5.

Does'nt look like Javascript has a pre-built function for this? Anybody know how to accomplish this?

I was thinking perhaps determinining what day of the year it is (1-365) and then dividing by 7. But I dont think javascript has a method for that either.

Thanks.

 

[wray]

This might work for you. Define Week 1 by plugging in the last day of that week then hand a date to WeekofYear() and get back a week number relative to Week 1.

  var gWeekMs  = 1000 * 60 * 60 * 24 * 7;

// initialize to Saturday 3 Jan 2004
  var gWeekOneMs = Date.UTC(2004,0,3);
  var gLastDay   = new Date(2004,0,3).getDay();

// calculate # days until end of week
function calcEndofWeek(dd) {
  var tLastDay = (gLastDay >= dd) ? gLastDay : gLastDay + 7;
  return (tLastDay - dd);
}

// calculate week of the year
function WeekofYear(m0,d,y) {
  tDate  = new Date(y,m0,d);
  tAdj   = calcEndofWeek(tDate.getDay());
  tAdjMs = Date.UTC(y,m0,d+tAdj);
  return ((tAdjMs - gWeekOneMs) / gWeekMs) + 1;
}

The Date() object has some quirks. Note that Month is specified relative 0, Day and Year relative 1. Also, the Date.UTC() calls sidestep a nasty Daylight Savings Time feature/bug.