How to convert week number and year into unix time

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

strtotime('1/1/2011 + 4 weeks') (1/1 ist always in week number one; this would bring me to week number five). if you want any timestamp in the week then that's all you need, else you would have to go to the monday in this week:

$t = strtotime('1/1/2011 + 4 weeks');
$t -=  24 * 60 * 60 * date('w', $t);

Update: Instead of 1/1/2011 use the first monday in 2011. The 2nd calculation is not needed anymore.

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.

Answer 3

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.