为什么Java和PHP之间的Epoch时间不同？

So I am porting code from PHP to Java. Part of the code has a time sensitive hashing algorithm, where the current Unix time stamp acts as a seed. I am running into the problem where my ported code is giving me different answers in either language. I suspect it could be a slight difference in implementation or something.

I am wondering if the problem is something rather obscure such as this. Nevertheless, any help would be greatly appreciated.

Here is my code in Java.

private static int generateB() {
    SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy", Locale.US);

    Date start;
    Date now;
    try {
        start = sdf.parse("01/01/1970");
        now = new Date();
    } catch (ParseException e) {
        return 402890;
    }
    long diff = now.getTime() - start.getTime();
    long hours = diff / (60 * 60 * 1000) % 24;
    long days = diff / (24 * 60 * 60 * 1000);
    return (int) (hours + days * 24);
}

and here is the PHP code:

 /**
 * Generate "b" parameter
 * The number of hours elapsed, since 1st of January 1970
 *
 * @return int
 */
private function generateB()
{
    $start = new \DateTime('1970-01-01');
    $now = new \DateTime('now');
    $diff = $now->diff($start);
    return $diff->h + ($diff->days * 24);
}

Yet they return different results, off by 3 hours to be exact.

At the time of this post. The PHP returns 403472 while Java returns 403475. Also if I use System.currentTimeMillis() I get 403480.

So my question is why is there any difference at all? I mean subtracting 3 could solve my problem but I am just curious for future reference why the difference exists at all. Note: I am running the PHP via PHP Sandbox for testing purposes.

Judging by your quote

At the time of this post. The PHP returns 403472 while Java returns 403475. Also if I use System.currentTimeMillis() I get 403480.

that all looks alright - there's 3 seconds between your PHP and Java, and 5 seconds on top of that. I guess that's probably the amount of time it took for you to switch process and run the tests?

php > echo date('Y-m-d H:i:s', 403472);
1970-01-05 17:04:32
php > echo date('Y-m-d H:i:s', 403475);
1970-01-05 17:04:35
php > echo date('Y-m-d H:i:s', 403480);
1970-01-05 17:04:40