Datetime Add/Subtract Miscalculation

Question

$datenow    = new DateTime();
$dn = $datenow -> format("Y-m-d"); //2014-12-02
$yesterday  = $datenow -> sub(new DateInterval('P1D')) -> format("Y-m-d"); //2014-12-01
$yestertwo  = $datenow -> sub(new DateInterval('P2D')) -> format("Y-m-d"); //2014-11-29
$tomorrow   = $datenow -> add(new DateInterval('P1D')) -> format("Y-m-d"); //2014-11-30
$tomotwo    = $datenow -> add(new DateInterval('P2D')) -> format("Y-m-d"); //2014-12-02

I had to be missing something here. Date calculation seems to be off.

Update:

$datenow            = new DateTime();
$dn            = $datenow -> format("Y-m-d");
$yesterday     = $datenow -> sub(new DateInterval('P1D')) -> format("Y-m-d");
$yestertwo     = $datenow -> sub(new DateInterval('P1D')) -> format("Y-m-d");
$tomorrow      = $datenow -> add(new DateInterval('P3D')) -> format("Y-m-d");
$tomotwo       = $datenow -> add(new DateInterval('P1D')) -> format("Y-m-d");

This outputs the correct date now. However, it looks kind of messy and unreadable at first glance. Any solutions?

Not really.. You just keep calculating with the recalculated date all the time.. — Naruto, Dec 02 '14 at 15:40

score 3 · Accepted Answer · answered Dec 02 '14 at 15:40

3

You're modifying $datenow each time you sub/add, so you're essentially changing what "today" means.

answered Dec 02 '14 at 15:40

ceejayoz

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@HDKT: Use `DateTimeImmutable` instead of `DateTime` and things _will_ go the way you expect. – Evert Dec 02 '14 at 15:55
@Evert You should post that as an answer. Thank you! – HDKT Dec 02 '14 at 15:58

score 1 · Answer 2 · answered Dec 02 '14 at 16:08

1

As @ceejayoz mentioned, when you call add or sub on a DateTime object, you also modify it.

Since PHP 5.5 there's a new class: DateTimeImmutable. This class has methods like add and sub as well, but instead of modifying the original, it just returns a new object with the modification applied.

Replace $datenow = new DateTime(); with $datenow = new DateTimeImmutable(); and things should just start working.

answered Dec 02 '14 at 16:08

Evert

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For php < 5.5 use `$dateclone = clone $datenow` to clone it, and then modify cloned object. – Glavić Dec 02 '14 at 16:13
I also tried the clone method but DateTimeImmutable works better for me. The added one line of code is a deal breaker for me. – HDKT Dec 04 '14 at 15:19

Datetime Add/Subtract Miscalculation

2 Answers2