Between dates : from highest -> current -> lowest

Question

For example I have 2 dates (year and month) like :

2013-03

2013-01 <-- Jan. current month.

2012-10

So my code is :

// Get current month and year
$m = date('m');
$y = date('Y');

// So I do something to get highest date so result is..
$highest = "2013-03";

// Current month and year
$current = $y."-".$m;


// So I do something to get lowest date so result is..
$lowest = "2012-10";


// My idea is get lower month from variables

function do_lower($date){
    $date = explode("-",$date);
    $y = $date[0];
    $m = $date[1];

    if($m != "01") {
        $m = $m-1;
        $m = sprintf('%02d',$m);
        return $y."-".$m;
    } else {
       return ($y-1)."-12";
    }

}


// This is my simple function to get result between high and low 

function different($h,$l) {
    global $data;
    $h = do_lower($h);
    if($h!=$l){
        $data[] = $h;
        different($h,$l);
    }
}

Then .. to do all functions like.. (JSON)

$data[] = $highest;
different($highest,$current);
$data[] = $current;
different($current,$lowest);
$data[] = $lowest;

So my result is correct

["2013-03","2013-02","2013-01","2012-12","2012-11","2012-10"]

Anyone can review my code ?

Thanks so much

Answer 1

There's not really much to review here since it's such a short snippet, but a few things:

---

I tend to avoid manipulating dates as strings. With just a year-month format it's actually fine since there's no odd rules. It's generally a lot safer to use DateTime though (or even strtotime/date). The problem with dates and times are that they are not simple units. There are complex, ever-changing 'business rules' (of sorts) that surround dates and times and cause huge headaches.

As said though, you're fine with year-date format since there's no gotcha's.

Just for consistency sake though, I would work with a standardized date format and use a date-time specific API.

For example:

$start = new DateTime(date('Y-m')); //First of this month
$end = new DateTime('2013-03'); //March 1st
$oneDay = new DateInterval('P1D');
$dates = array();
for (; $end >= $start; $end->sub($oneDay)) {
    $dates[] = $end->format('Y-m');
}

$dates is then an array in the format you want. This code is quite ugly, but unfortunately I've never been able to figure out a way to manipulate dates in PHP that isn't ugly.

---

Here's a strtotime/date based version:

$start = strtotime(date('Y-m'));
$end = strtotime('2013-03');
$dates = array();
while ($end >= $start) {
    $dates[] = date('Y-m', $end);
    $end = strtotime(date('Y-m-d', $end) . ' +1 day');
}

Rather than the strtotime call, you could just use 864000 seconds. I'd have to think for a few minutes to make sure that won't have any DST mistakes though. Once again, a situation where I like to keep date operations as abstracted away as possible. 864000 is a lie since not all days have 24 hours (at least not in places that observe DST).

---

If I were to go with a string approach, your do_lower function is basically what I'd use. It's named rather poorly though since it has no mention of what it's 'lowering' or what 'lowering' even means. Also, I would probably use a strict comparison against 01.

The problem with a purely string based approach is that it's not very reusable. Your function has a very specific input format and a very specific output format. Anything else and it breaks.

If you pass around DateTime's instead, dates are no longer represented as a string. They're represented as a type that is much more appropriate for such a complicated system. They're represented as a type that is aware that they are a date.