Question

I am having a problem with php’s DateTime functions.
Today is monday 3 december.

Assuming the following code:

$dte = new DateTime(date('Y-m-d H:i:s'));
var_dump($dte->format('Y-W'));
$dte->modify('+4 weeks');
var_dump($dte->format('Y-m-d H:i:s -- Y_W'));
$dte->modify('+1 days');
var_dump($dte->format('Y-m-d H:i:s -- Y_W'));

After four weeks it would be 31st of december. I would suspect to get the last week of the year (52?). But what I get is week 1 of 2012 as you can see in the following output.

string '2012-49' (length=7)
string '2012-12-31 14:48:00 -- 2012_01' (length=30)
string '2013-01-01 14:48:00 -- 2013_01' (length=30)

So my problem is that after the first modification I think I should get:

2012-12-31 14:48:00 -- 2012_52

but instead I get

2012-12-31 14:48:00 -- 2012_01

So why does the week go back to 01 without incrementing the year, and than why does the other line gives me 2013_01 ?

EDIT::
I now see that the week before is week 52, anything to do with leap year?
But then again, how can the week go back to 01 without incrementing the year?

Answer 1

So why does the week go back to 01 without incrementing the year, and than why does the other line gives me 2013_01 ?

I think you’re displaying the “year” instead of the “week-year”. When you’re using week numbers, it’s the week-year that’s the relevant part; simple “year” is only relevant with respect to month and day.

EDIT: I think you want the o format specifier instead, so try:

var_dump($dte->format('Y-m-d H:i:s -- o_W'));

That should show you 2013_01 for December 31st 2012, as it’s in week 1 of week-year 2013.

So basically, I don’t think this is a bug in DateTime – it’s just a misunderstanding of how “week of year” is meant to be used.