TimeframeForm: Fix DateTime handling

- `DateTime::createFromFormat()` returns false, if seconds are `00`, which leads to a false positive result at the end.
- Check the checkboxes only if start/end is actually relative

library/Reporting/Web/Forms/TimeframeForm.php

@@ -46,16 +46,10 @@ protected function assemble()
             'description' => $this->translate('A unique name of this timeframe')
         ]);
 
-        $default = new DateTime('00:00:00');
-        $start = $this->getPopulatedValue('start', $default);
-        if (! $start instanceof DateTime) {
-            $datetime = DateTime::createFromFormat(LocalDateTimeElement::FORMAT, $start);
-            if ($datetime) {
-                $start = $datetime;
-            }
-        }
-
-        $relativeStart = $this->getPopulatedValue('relative-start', $start instanceof DateTime ? 'n' : 'y');
+        $start = $this->getPopulatedValue('start', new DateTime('00:00:00'));
+        $canBeConverted = $start instanceof DateTime
+            || DateTime::createFromFormat(LocalDateTimeElement::FORMAT, $start) !== false;
+        $relativeStart = $this->getPopulatedValue('relative-start', $canBeConverted ? 'n' : 'y');
         $this->addElement('checkbox', 'relative-start', [
             'required' => false,
             'class'    => 'autosubmit',
@@ -65,7 +59,7 @@ protected function assemble()
 
         if ($relativeStart === 'n') {
             if (! $start instanceof DateTime) {
-                $start = $default;
+                $start = (new DateTime($start))->format(LocalDateTimeElement::FORMAT);
                 $this->clearPopulatedValue('start');
             }
 
@@ -102,16 +96,10 @@ protected function assemble()
             ]);
         }
 
-        $default = new DateTime('23:59:59');
-        $end = $this->getPopulatedValue('end', $default);
-        if (! $end instanceof DateTime) {
-            $datetime = DateTime::createFromFormat(LocalDateTimeElement::FORMAT, $end);
-            if ($datetime) {
-                $end = $datetime;
-            }
-        }
-
-        $relativeEnd = $this->getPopulatedValue('relative-end', $end instanceof DateTime ? 'n' : 'y');
+        $end = $this->getPopulatedValue('end', new DateTime('23:59:59'));
+        $canBeConverted = $end instanceof DateTime
+            || DateTime::createFromFormat(LocalDateTimeElement::FORMAT, $end) !== false;
+        $relativeEnd = $this->getPopulatedValue('relative-end', $canBeConverted ? 'n' : 'y');
         if ($relativeStart === 'y') {
             $this->addElement('checkbox', 'relative-end', [
                 'required' => false,
@@ -153,7 +141,7 @@ protected function assemble()
 
         if ($relativeEnd === 'n' || $relativeStart === 'n') {
             if (! $end instanceof DateTime) {
-                $end = $default;
+                $end = (new DateTime($end))->format(LocalDateTimeElement::FORMAT);
                 $this->clearPopulatedValue('end');
             }