Looks to me that you mean a string, not an array.
To convert that to an array, you'd simply wrap it in
array(). So that would be
array($start_year, . . ., $start_seconds);.
What you want is a string. I'd recommend a sprintf as its simple.
Since these come from HTML, you can also generate the array there. That may be a lot easier to work with and be more beneficial overall.
$start_time = sprintf('%d %d %d %d %d %d', $start_year, $start_month, $start_date, $start_hour, $start_minute, $start_seconds);
Then it will be retrieved as an array already. This is nice as to match the format you have above you'd simply implode() with a space, but you can still look up what you need based on the offset of $_POST['inputDate']['itemhere'].
<input type="text" name="inputDate[year]" />
<input type="text" name="inputDate[month]" />
BTW, if you split up the date from the time strings, you can parse it easily using strtotime or new DateTime objects. This makes it easier to deal with other calculations based on time as well as letting you format it however you want.
For example. strtotime() and date() can be used in place of datetime objects.
// input types date and time
$sDateStr = implode('', $_POST['inputDate']);
$sTimeStr = implode('.', $_POST['inputTime']);
$dt = new DateTime("$sDateStr $sTimeStr");
print $dt->format('Y-m-d H:i:s');