Add first handoff hint in the display timezone

Show the first handoff in the display timezone if it differs to the
schedule timezone.

Author: jrauh01

application/forms/RotationConfigForm.php

@@ -1242,7 +1242,7 @@ function ($value, $validator) use ($earliestHandoff, $firstHandoff, $latestHando
                     if ($actualFirstHandoff < new DateTime()) {
                         return $this->translate('The rotation will start immediately');
                     } else {
-                        $handoffHint = sprintf(
+                        return sprintf(
                             $this->translate('The rotation will start on %s'),
                             (new \IntlDateFormatter(
                                 \Locale::getDefault(),
@@ -1251,15 +1251,37 @@ function ($value, $validator) use ($earliestHandoff, $firstHandoff, $latestHando
                                 $this->scheduleTimezone
                             ))->format($actualFirstHandoff)
                         );
-
-                        if ($this->displayTimezone !== $this->scheduleTimezone) {
-                            $handoffHint .= sprintf($this->translate(' (in %s)'), $this->scheduleTimezone);
-                        }
-
-                        return $handoffHint;
                     }
                 })
             ));
+            if ($this->displayTimezone !== $this->scheduleTimezone) {
+                $this->addHtml(new HtmlElement(
+                    'p',
+                    Attributes::create(['id' => 'first-handoff-display-timezone-hint']),
+                    DeferredText::create(function () {
+                        $ruleGenerator = $this->yieldRecurrenceRules(1);
+                        if (! $ruleGenerator->valid()) {
+                            return '';
+                        }
+
+                        $actualFirstHandoff = $ruleGenerator->current()[0]->getStartDate();
+                        if ($actualFirstHandoff < new DateTime()) {
+                            return '';
+                        } else {
+                            return sprintf(
+                                $this->translate('This would be %s in the chosen display timezone (%s)'),
+                                (new \IntlDateFormatter(
+                                    \Locale::getDefault(),
+                                    \IntlDateFormatter::MEDIUM,
+                                    \IntlDateFormatter::SHORT,
+                                    $this->displayTimezone
+                                ))->format($actualFirstHandoff),
+                                $this->displayTimezone,
+                            );
+                        }
+                    })
+                ));
+            }
         }
 
         $this->addElement('submit', 'submit', [

public/css/form.less

@@ -70,7 +70,8 @@
     align-items: baseline;
   }
 
-  #first-handoff-description {
+  #first-handoff-description,
+  #first-handoff-display-timezone-hint {
     margin: 0 0 0 14em;
   }