How to convert week number and year into unix timestamp?

Question

I'm trying to group together dates into a week number and year, and then I want to convert that week number back into a unix timestamp. How can I go about doing this?

Answer 1

I assume you are using ISO 8601 week numbers, and want the first day of a ISO 8601 week so that e.g. Week 1 of 2011 returns January 3 2011.

strtotime can do this out of the box using the {YYYY}W{WW} format:

echo date("Y-m-d", strtotime("2011W01")); // 2011-01-03 

Note that the week number needs to be two digits.

Shamefully, DateTime::createFromFormat, the fancy new PHP 5 way of dealing with dates, seems unable to parse this kind of information - it doesn't have a "week" placeholder.

Answer 2

• $week: The week number
• $year: The year number

Then:

$timestamp = gmmktime (0, 0 , 0 , 1, , 4 + 7*($week - 1), $year); 

The 4 + 7*($week - 1) comes from the fact that according to ISO 8601, the first week of the year is the one that contains January 4th.