Unit testing date sensitive functions?

I am trying to write tests for a date helper class. I am wondering what is best practice for this.

Here is an example in PHP.

    public static function get_date($offset = 0){
        $date = mktime(0, 0, 0, date('m')  , date('d') + $offset, date('Y'));
        return array(
            'day'   => date('D',$date),
            'month' => date('M',$date),
            'year'  => date('Y',$date)
        );
    }

    public static function today(){
        return self::get_date();
    }

    public static function tomorrow(){
        return self::get_date(1);
    }

    public static function yesterday(){
        return self::get_date(-1);
    }

So what I am looking for is examples of tests that could test these functions or a new way to write these functions so they are开发者_运维技巧 intuitively testable.

I have found examples in Java but they seem pretty inelegant and I really don't know Java, unless you count Actionscript 3 as Java ;)

Solutions in Javascript or Ruby would also be super helpful as examples but my class is written in PHP so that would be ideal.

Thanks!

---

I usually do something like this:

<?php

class Date
{
    protected $date;

    public function __construct($date = null)
    {
        $this->date = $date ? $date : time();
    }

    public function today()
    {
        // to something with $this->date
    }
}


class DateTest extends PHPUnit_Framework_TestCase
{
    public function testToday()
    {
        $date = new Date(strtotime('2010-10-17 00:00:00'));
        $this->assertEquals($expectedValue, $date->today());
    }
}

It follows the same principle that date() does, which is to receive a second optional argument, the reference time.