Xls Writer - Correct Timestamp Bug, Improve Coverage (#1493)

* Xls Writer - Correct Timestamp Bug, Improve Coverage

I believe that Xls Writer is 100% covered now.

The Xls Writer sets its timestamp incorrectly. The problem is actually
in Shared/Ole::localDateToOLE, which converts its timestamp using
gmmktime; mktime is correct. If I save a file at 3:00 p.m. in San Francisco,
this bug means the time is actually recorded as 3:00 p.m. UTC.
A consequence of this is that if you use Phpspreadsheet to read the
file and save it as a new Xls, the creation timestamp goes further
and further back in time with each generation (or further forward
if east of Greenwich). One of the tests added confirms that
the creation timestamp is consistent with the start and end times
of the test.

The major change in coverage is adding tests to save GIF and BMP
images, which aren't supported in Xls, but are converted to PNG
in the PhpSpreadsheet code.

src/PhpSpreadsheet/Shared/OLE.php

@@ -505,7 +505,7 @@ public static function localDateToOLE($date)
         // days from 1-1-1601 until the beggining of UNIX era
         $days = 134774;
         // calculate seconds
-        $big_date = $days * 24 * 3600 + gmmktime(date('H', $date), date('i', $date), date('s', $date), date('m', $date), date('d', $date), date('Y', $date));
+        $big_date = $days * 24 * 3600 + mktime((int) date('H', $date), (int) date('i', $date), (int) date('s', $date), (int) date('m', $date), (int) date('d', $date), (int) date('Y', $date));
         // multiply just to make MS happy
         $big_date *= 10000000;
 
@@ -558,10 +558,9 @@ public static function OLE2LocalDate($oleTimestamp)
         // translate to seconds since 1970:
         $unixTimestamp = floor(65536.0 * 65536.0 * $timestampHigh + $timestampLow - $days * 24 * 3600 + 0.5);
 
-        if ((int) $unixTimestamp == $unixTimestamp) {
-            return (int) $unixTimestamp;
-        }
+        $iTimestamp = (int) $unixTimestamp;
 
-        return $unixTimestamp >= 0.0 ? PHP_INT_MAX : PHP_INT_MIN;
+        // Overflow conditions can't happen on 64-bit system
+        return ($iTimestamp == $unixTimestamp) ? $iTimestamp : ($unixTimestamp >= 0.0 ? PHP_INT_MAX : PHP_INT_MIN);
     }
 }