date to week number, and week number to date (monday), in js and erlang

example.erl

% there are 52-53 weeks in the year
% http://www.whatweekisit.org/calendar-2020.html


% date to week number in year
> calendar:iso_week_number({2020,1,1}).
{2020,1}
> calendar:iso_week_number({2019,12,31}).
{2020,1}


% get monday (date) of the week, for any day (date) from that week
% Date = {Year, Month, Day}
monday_in_the_week_of(Date) ->
  V1 = calendar:date_to_gregorian_days(Date) - (calendar:day_of_the_week(Date) - 1),
  calendar:gregorian_days_to_date(V1).


% test
> m1:monday_in_the_week_of({2020,1,1}).
{2019,12,30}
> m1:monday_in_the_week_of({2020,1,2}).
{2019,12,30}
> m1:monday_in_the_week_of({2020,1,3}).
{2019,12,30}
> m1:monday_in_the_week_of({2020,1,4}).
{2019,12,30}
> m1:monday_in_the_week_of({2020,1,5}).
{2019,12,30}
> m1:monday_in_the_week_of({2020,1,6}).
{2020,1,6}


% get monday (date) of the week, by week number and year
% Week_Num = 1 .. 53
% check 53th week in year exists = if date for 53th week is equal to 52th week = means max week is 52th
monday_in_the_week_of_year(Week_Num, Year) when Week_Num > 0, Week_Num < 54 ->
  Month = if Week_Num < 8 -> 1;
             true ->
               V1 = erlang:floor(Week_Num / 4),
               if V1 > 12 -> 12; true -> V1 end
          end,
  Day = 1,
  ?MODULE:monday_in_the_week_of_year_helper({Year, Month, Day}, Week_Num, Year).

monday_in_the_week_of_year_helper({Year, Month, Day}=Date, Week_Num, Year) ->
  Date2 = calendar:gregorian_days_to_date( calendar:date_to_gregorian_days(Date) - (calendar:day_of_the_week(Date) - 1) ),
  {Year3, Week_Num2} = calendar:iso_week_number(Date2),
  if Year3 =:= Year, Week_Num2 =:= Week_Num ->
      Date2;
    true ->
      Max_Day = calendar:last_day_of_the_month(Year, Month),
      Day3 = Day + 7,
      if (Day3 > Max_Day) ->
          case Month of
            12 -> calendar:gregorian_days_to_date( calendar:date_to_gregorian_days({Year, 12, Max_Day}) - (calendar:day_of_the_week({Year, 12, Max_Day}) - 1) );
            _ -> ?MODULE:monday_in_the_week_of_year_helper({Year, Month + 1, Day3 - Max_Day}, Week_Num, Year)
          end;
        true -> ?MODULE:monday_in_the_week_of_year_helper({Year, Month, Day3}, Week_Num, Year)
      end
  end.


% test
> m1:monday_in_the_week_of_year(1, 2020). 
{2019,12,30}
> m1:monday_in_the_week_of_year(2, 2020). 
{2020,1,6}
> m1:monday_in_the_week_of_year(3, 2020). 
{2020,1,13}
> m1:monday_in_the_week_of_year(25, 2020).
{2020,6,15}
> m1:monday_in_the_week_of_year(52, 2020).
{2020,12,21}
> m1:monday_in_the_week_of_year(53, 2020).
{2020,12,28}

> m1:monday_in_the_week_of_year(5, 2019). 
{2019,1,28}
> m1:monday_in_the_week_of_year(52, 2019).
{2019,12,23}
> m1:monday_in_the_week_of_year(53, 2019).
{2019,12,30}

> m1:monday_in_the_week_of_year(1, 2021). 
{2021,1,4}
> m1:monday_in_the_week_of_year(2, 2021).
{2021,1,11}
> m1:monday_in_the_week_of_year(3, 2021).
{2021,1,18}
> m1:monday_in_the_week_of_year(50, 2021).
{2021,12,13}
> m1:monday_in_the_week_of_year(51, 2021).
{2021,12,20}
> m1:monday_in_the_week_of_year(52, 2021).
{2021,12,27}
> m1:monday_in_the_week_of_year(53, 2021).
{2021,12,27}

example.js

// there are 52-53 weeks in the year
//http://www.whatweekisit.org/calendar-2020.html


//date to week number in year
function getWeekNumber(d) {
  d = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
  d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay()||7));
  var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
  var weekNo = Math.ceil(( ( (d - yearStart) / 86400000) + 1)/7);
  return [d.getUTCFullYear(), weekNo];
}


// test
getWeekNumber(new Date('2020/1/1'))
[2020,1]
getWeekNumber(new Date('2019/12/31'))
[2020,1]



// week number and year to week's date (monday)
function getDateOfMondayWeek(w, y){
  var simple = new Date(y, 0, 1 + (w - 1) * 7);
  var dow = simple.getDay();
  var ISOweekStart = simple;
  if(dow <= 4){
    ISOweekStart.setDate(simple.getDate() - simple.getDay() + 1);
  }else{
    ISOweekStart.setDate(simple.getDate() + 8 - simple.getDay());
  }
  return ISOweekStart;
}


// test
getDateOfMondayWeek(1,2020);
Date Mon Dec 30 2019 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

getDateOfMondayWeek(52,2019);
Date Mon Dec 23 2019 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

getDateOfMondayWeek(2,2020);
Date Mon Jan 06 2020 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

getDateOfMondayWeek(51,2021)
Date Mon Dec 20 2021 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

getDateOfMondayWeek(52,2021)
Date Mon Dec 27 2021 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

getDateOfMondayWeek(53,2021)
Date Mon Jan 03 2022 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

getDateOfMondayWeek(1,2022)
Date Mon Jan 03 2022 00:00:00 GMT+0200 (за східноєвропейським стандартним часом)

//check for 53th week of year exists -- compare with 1st week of next year -- if the same = no 53th week