Comparing two ISO8601 dates strings in PHP

Question

I need to compare (actually rank/sort) dates in a PHP script. The dates are ISO-8601 Date format i.e.

YYYY-MM-DD

I wrote a comparison function which splits the dates and compares by year/month/day. However, it seems this may be overkill and I could just as easily done a simple string comparison like:

if ($date1 < $date2)
   // do something
elseif( $date1 > $date2)
   //do something else
else
   //do yet another thing

Is my assumption about (ISO-8601) Date string comparison correct - i.e. can I get rid of my function (to save a few clock cycles on the server), or is it safer to explicity do the comparison in a custom function?

Answer 1

http://en.wikipedia.org/wiki/ISO_8601#General_principles

Date and time values are organized from the most to the least significant: year, month (or week), day, hour, minute, second, and fraction of second. The lexicographical order of the representation thus corresponds to chronological order, except for date representations involving negative years. This allows dates to be naturally sorted by, for example, file systems.

Go ahead with string sorting. If wikipedia is not enough, then surely http://www.ietf.org/rfc/rfc3339.txt is, search for strcmp in there.

Answer 2

If you turn your dates into DateTime objects (Usually available from PHP 5.2+), you can reliably use comparisons regardless of format.

$date = new DateTime("YYYY-MM-DD");
$date2 = new DateTime("YYYY-MM-DD");

if ($date > $date2)  .....