How to find week of month for calendar which starts from Monday?

Question

I have a calendar starting on Monday.

May, 2011

----------------------------
Mon Tue Wed Thu Fri Sat Sun
----------------------------
25  26  27  28  29  30   1     week 1
2   3   4    5   6   7   8     week 2
9   10  11  12  13  14   15    week 3
16  17  18  19  20  21   22    week 4
23  24  25  26  27  28   29    week 5
30  31  1   2    3  4     5    week 6

I want to find week of month on giving Date.

Below works fine on the calendar that starts on Sunday.

function getWeekOfMonth(date) {

  prefixes = ['1', '2', '3', '4', '5'];
  return prefixes[0 | date.getDate() / 7];
}

But in my Calendar (Monday based) if I choose May 1st, the return value if "1" thus gives me "week2" but it should be "week1".

If I choose May 15th, giving me "week4" instead of "week3".

It works on things like May 12th.

I tried to shift this one day gap (monday and sunday) and modified the algorithm which failed.

Could anyone show me how can I fix my algorithm correctly?

function getWeekOfMonth(date) {

  prefixes = ['1', '2', '3', '4', '5'];
  return prefixes[0 | date.getDate() - 1 / 7];
}

Answer 1

you first need to check on what day the month starts then add k so that date.getDate() + k is 7 for the first monday of the month

then it's pretty easy

function getWeekOfMonth(date) {
  var adjustedDate = date.getDate()+k;
  prefixes = ['0', '1', '2', '3', '4', '5'];
  return prefixes[0 | adjustedDate / 7];
}

---

edit:

you can use getDay to find the current day of week and use it in relation to getDate to find the monday of the current week

function getWeekOfMonth(date) {
  var day = date.getDate()
  day-=(date.getDay()==0?6:date.getDay()-1);//get monday of this week
  //special case handling for 0 (sunday)

  day+=7;
  //for the first non full week the value was negative

  prefixes = ['0', '1', '2', '3', '4', '5'];
  return prefixes[0 | (day) / 7];
}