Carry a numeric rounding precision out of any unit, not just seconds

src/Carbon/Traits/Rounding.php

@@ -70,21 +70,35 @@ trait Rounding
 
         $precision *= $factor;
 
-        // A numeric precision is in seconds, so once it spans the next coarser
-        // unit it must be re-targeted there, like "week" -> "day" above.
-        if ($normalizedUnit === 'second' && (\is_int($precision) || \is_float($precision))) {
-            $secondsPerHour = static::MINUTES_PER_HOUR * static::SECONDS_PER_MINUTE;
-            $secondsPerDay = static::HOURS_PER_DAY * $secondsPerHour;
+        // How many of each unit make up the next one up. Only the steps with a
+        // fixed size are listed: a month holds a varying number of days, so a
+        // day precision is never carried into a month.
+        $unitsPerNextUnit = [
+            // @call roundUnit
+            'microsecond' => [static::MICROSECONDS_PER_SECOND, 'second'],
+            // @call roundUnit
+            'second' => [static::SECONDS_PER_MINUTE, 'minute'],
+            // @call roundUnit
+            'minute' => [static::MINUTES_PER_HOUR, 'hour'],
+            // @call roundUnit
+            'hour' => [static::HOURS_PER_DAY, 'day'],
+            // @call roundUnit
+            'month' => [static::MONTHS_PER_YEAR, 'year'],
+        ];
 
-            if ($precision >= $secondsPerDay) {
-                $normalizedUnit = 'day';
-                $precision /= $secondsPerDay;
-            } elseif ($precision >= $secondsPerHour) {
-                $normalizedUnit = 'hour';
-                $precision /= $secondsPerHour;
-            } elseif ($precision >= static::SECONDS_PER_MINUTE) {
-                $normalizedUnit = 'minute';
-                $precision /= static::SECONDS_PER_MINUTE;
+        // A numeric precision counts units of $normalizedUnit, and that unit has
+        // a range it cannot leave, so a precision spanning the next unit up has
+        // to be re-targeted there, the same way "week" becomes "day" above.
+        if (\is_int($precision) || \is_float($precision)) {
+            while (isset($unitsPerNextUnit[$normalizedUnit])) {
+                [$span, $nextUnit] = $unitsPerNextUnit[$normalizedUnit];
+
+                if ($precision < $span) {
+                    break;
+                }
+
+                $precision /= $span;
+                $normalizedUnit = $nextUnit;
             }
         }