Cron/OneOff: Always use the application's time zone

Since `Cron` & `OneOff` frequencies work with `DatemTime` by either
calling the methods of the class manually or constructing from a
serialized representation, time zones can differ from the application's
time zone. However, in the frontend we don't display time zone
information, so the time may appear incorrect. This change ensures that
both frequencies always work with the application's time zone as well.

src/Cron.php

@@ -5,6 +5,7 @@
 use Cron\CronExpression;
 use DateTime;
 use DateTimeInterface;
+use DateTimeZone;
 use InvalidArgumentException;
 use ipl\Scheduler\Contract\Frequency;
 
@@ -98,6 +99,7 @@ public function getExpression(): string
     public function startAt(DateTimeInterface $start): self
     {
         $this->start = clone $start;
+        $this->start->setTimezone(new DateTimeZone(date_default_timezone_get()));
 
         return $this;
     }
@@ -112,6 +114,7 @@ public function startAt(DateTimeInterface $start): self
     public function endAt(DateTimeInterface $end): Frequency
     {
         $this->end = clone $end;
+        $this->end->setTimezone(new DateTimeZone(date_default_timezone_get()));
 
         return $this;
     }

src/OneOff.php

@@ -4,17 +4,18 @@
 
 use DateTime;
 use DateTimeInterface;
-use InvalidArgumentException;
+use DateTimeZone;
 use ipl\Scheduler\Contract\Frequency;
 
 class OneOff implements Frequency
 {
     /** @var DateTime Start time of this frequency */
     protected $dateTime;
 
-    public function __construct(DateTime $dateTime)
+    public function __construct(DateTimeInterface $dateTime)
     {
         $this->dateTime = clone $dateTime;
+        $this->dateTime->setTimezone(new DateTimeZone(date_default_timezone_get()));
     }
 
     public function isDue(DateTimeInterface $dateTime): bool