Implement resolveUTC

(cherry picked from commit e4089a9)

src/Carbon/Traits/Date.php

@@ -737,6 +737,30 @@ protected function resolveCarbon($date = null)
         return $date instanceof self ? $date : static::instance($date);
     }
 
+    /**
+     * Return the Carbon instance passed through, a now instance in UTC
+     * if null given or parse the input if string given (using current timezone
+     * then switching to UTC).
+     *
+     * @param Carbon|DateTimeInterface|string|null $date
+     *
+     * @return static
+     */
+    protected function resolveUTC($date = null): self
+    {
+        if (!$date) {
+            return static::now('UTC');
+        }
+
+        if (\is_string($date)) {
+            return static::parse($date, $this->getTimezone())->utc();
+        }
+
+        static::expectDateTime($date, ['null', 'string']);
+
+        return $date instanceof self ? $date : static::instance($date)->utc();
+    }
+
     /**
      * Return the Carbon instance passed through, a now instance in the same timezone
      * if null given or parse the input if string given.

src/Carbon/Traits/Difference.php

@@ -638,7 +638,7 @@ public function floatDiffInRealHours($date = null, $absolute = true)
      */
     public function floatDiffInRealDays($date = null, $absolute = true)
     {
-        $date = $this->resolveCarbon($date)->utc();
+        $date = $this->resolveUTC($date);
         $utc = $this->copy()->utc();
         $hoursDiff = $utc->floatDiffInRealHours($date, $absolute);
 

tests/Carbon/DiffTest.php

@@ -1606,6 +1606,13 @@ public function testFloatDiff()
 
     public function testFloatDiffWithRealUnits()
     {
+        $from = Carbon::parse('2021-03-27 20:00 Europe/Warsaw');
+        $to = Carbon::parse('2021-03-27 20:00 Europe/London');
+        $from->floatDiffInRealDays($to);
+
+        $this->assertSame('2021-03-27 20:00:00 Europe/Warsaw', $from->format('Y-m-d H:i:s e'));
+        $this->assertSame('2021-03-27 20:00:00 Europe/London', $to->format('Y-m-d H:i:s e'));
+
         date_default_timezone_set('UTC');
 
         $this->assertSame(0.9583333333333335, Carbon::parse('2021-03-27 20:00 Europe/Warsaw')->floatDiffInRealDays('2021-03-28 20:00'));