Support unix timestamps with precision lower than 6 digits

[preinheimer]

Creating a new DateTime with four digits of precision gives an unexpected result.

$time = new DateTimeImmutable('@' . '1508765076.3470');
echo $time->format('r') . "\n";
// Wed, 24 Oct 3517 13:24:36 +0000

$time = new DateTimeImmutable('@' . '1508765076.347000');
echo $time->format('r') . "\n";
// Mon, 23 Oct 2017 13:24:36 +0000

$time = new DateTimeImmutable('@' . '1508765076'); 
echo $time->format('r') . "\n";
// Mon, 23 Oct 2017 13:24:36 +0000

Currently only 6 digits of precision are supported. It seems as though supporting other levels of precision would be helpful.

[derickr]

I've fixed this for 1-6 digits precision. timelib's resolution isn't higher than µsec, so I can't support more than 6 digits (without discarding the result).