Question

I need to display the current week in a calendar view, starting from Sunday.

What’s the safest way to determine “last sunday” in Javascript?

I was calculating it using the following code:

Date.prototype.addDays = function(n) {
      return new Date(this.getTime() + (24*60*60*1000)*n);
}

var today = new Date(now.getFullYear(), now.getMonth(), now.getDate());
var lastSunday = today.addDays(0-today.getDay());

This code makes the assumption that every day consists of twenty four hours. This is correct, EXCEPT if it’s a daylight savings crossover day, in which case the day could be twenty-three or twenty-five hours.

This week, In Sydney, Australia, we set our clocks forward an hour. As a result, my code calculates lastSunday as 23:00 on Saturday.

So what IS the safest and most efficient way to determine last Sunday?

Answer 1

To safely add exactly one day, use:

d.setDate(d.getDate() + 1);

which is daylight saving safe. To set a date object to the last Sunday:

function setToLastSunday(d) {
  return d.setDate(d.getDate() - d.getDay());
}

Or to return a new Date object for last Sunday:

function getLastSunday(d) {
  var t = new Date(d);
  t.setDate(t.getDate() - t.getDay());
  return t;
}

Edit

The original answer had an incorrect version adding time, that does add one day but not how the OP wants.