From week number to start and end timestamp

[nicolasr75]

A common scenario in my business domain is that users select a calendar week by selecting the week number and the year. The software is supposed to convert this into the start and end timestamps for the given week. An example would be a query for all measured temperature values of a specific week.

Temporal knows weekOfYear and dayOfWeek but how do I go the other direction?
Given an ISO calendar week number and a year, how do we get to the start and end date of that week?

moment.js allows me to set the week, see f.e. here:
https://www.tutorialspoint.com/momentjs/momentjs_week_of_year.htm

[justingrant]

Hi @nicolasr75 - Below is one way to get the date range you're looking for. The trick is that the fourth of January is always on the first ISO week of the year. So to calculate a week range, just find the start of the week that contains Jan 4, and add the correct number of weeks.

function getWeekRange({weekNumber, year}) {
  const dayInFirstWeek = Temporal.PlainDate.from({ year, month: 1, day: 4 });
  const start = dayInFirstWeek.add({ days: 1 - dayInFirstWeek.dayOfWeek }).add({ weeks: weekNumber - 1 });
  const end = start.add({ days: 6 });
  return { start, end };
}

function formatDate(date) {
  const formatted = date.toLocaleString(undefined, { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' });
  return `${formatted} (week ${date.weekOfYear})`
}
const { start, end } = getWeekRange({year: 2022, weekNumber: 2});
console.log(`${formatDate(start)} - ${formatDate(end)}`);
// => Monday, January 10, 2022 (week 2) - Sunday, January 16, 2022 (week 2)

EDIT 2022-01-28: fixed a bug in code above as discussed in later comments

[nicolasr75]

Thanks, I see. Would be great to see that example in the cookbook or somewhere else. Not everyone will think of this immediately ;-)

[ptomato]

Would you be able to mention a bit more about your use case in js-temporal/proposal-temporal-v2#11 ?

[nicolasr75]

@ptomato Done!

[nicolasr75]

@justingrant Your example does not work for week number 1 in 2022 because

const start = dayInFirstWeek.add({days: 1 - dayInFirstWeek.dayOfWeek, weeks: weekNumber - 1});

throws an error "mixed-sign values not allowed as duration fields". That is because days is -1 in this case and weeks is 0.
It works when I split it up into:

const start dayInFirstWeek.add({weeks: weekNumber - 1}).add({ days: 1 - dayInFirstWeek.dayOfWeek });

[justingrant]

@nicolasr75 - Thanks for catching that! Temporal.Duration instances cannot have opposite signs (see #782). Because add and subtract operate on durations, the same limitation also applies to property bags used in place of a Temporal.Duration instance.

Note that week number 1 is OK because zeroes are always allowed. It's only mixing positive and negative that fails. So week number 2+ will fail, e.g. getWeekRange({year: 2022, weekNumber: 2});.

I updated the code in my original comment so that folks finding this in Google won't copy/paste the bug.

[ptomato]

I think the question has been answered, so I'll close this.