Calculate number of specific weekdays between dates

Question

I'm trying to calculate the number of Mondays, Wednesdays, and Fridays between 2 dates in Tasker, thus I need a math formula, possibly utilizing the date in seconds form, i.e. unix time, or a javascript code. I've tried Googling and racking my brain for any way to even start this and I'm lost so I haven't tried anything yet. The only thing I could think of was getting the total number of days and dividing by 7, but that clearly does not help me very much, especially if one or both of the days is midweek. Can anyone point me in a better direction?

Answer 1

How to count specific days of the week between two dates in O(1):

// days is an array of weekdays: 0 is Sunday, ..., 6 is Saturday
function countCertainDays( days, d0, d1 ) {
  var ndays = 1 + Math.round((d1-d0)/(24*3600*1000));
  var sum = function(a,b) {
    return a + Math.floor( ( ndays + (d0.getDay()+6-b) % 7 ) / 7 ); };
  return days.reduce(sum,0);
}

Example on counting Mondays, Wednesdays, and Fridays [1,3,5] between two dates:

countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,1)) // 1
countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,2)) // 1
countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,3)) // 2
countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,4)) // 2
countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,5)) // 3
countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,6)) // 3
countCertainDays([1,3,5],new Date(2014,8,1),new Date(2014,8,7)) // 3

Note that the month parameter to Date is 0-based, so 1 Sept 2014 is Date(2014,8,1).

Answer 2

Assume total number of days is n. Then number of any day of a week is initially n / 7. Now take n = n % 7. Depending on the value of current n you can easily calculate the final count of days.

As example:

Assume your first day is Friday and total number of days is 100. So, n = 100. There are minimum 100/7 or 14 of each weekday is in the interval. 100 % 7 = 2. So final count is,

Friday -> 14+1 = 15
Saturday -> 14+1 = 15
Sunday -> 14
Monday -> 14
Tuesday -> 14
Wednesday -> 14
Thursday -> 14