Configuration.php

private function getFrequencySection()
    {
        $builder = new TreeBuilder();
        $node = $builder->root('frequencies');
        return $node
            ->cannotBeEmpty()
            ->requiresAtLeastOneElement()
            ->prototype('array')
                ->beforeNormalization()
                    ->ifString()
                    ->then(function($v){ return [ 'interval' => $v ]; })
                ->end()
                ->isRequired()
                ->addDefaultsIfNotSet()
                ->children()
                    ->scalarNode('interval')
                        ->validate()
                            ->ifTrue(function($v) { return !$this->intervalStringIsValid($v); })
                            ->thenInvalid("Invalid DateInterval string detected: %s. Check http://php.net/manual/en/dateinterval.createfromdatestring.php#refsect1-dateinterval.createfromdatestring-examples")
                        ->end()
                        ->isRequired()
                        ->cannotBeEmpty()
                        ->defaultValue('P1D')
                    ->end()
                    ->integerNode('times')
                        ->info('Number of times to repeat this interval')
                        ->defaultValue(1)
                    ->end()
                    ->scalarNode('rand_start')
                        ->defaultValue(0)
                        ->validate()
                            ->ifTrue(function($v) { return !$this->intervalStringIsValid($v); })
                            ->thenInvalid("Invalid DateInterval string detected: %s. Check http://php.net/manual/en/dateinterval.createfromdatestring.php#refsect1-dateinterval.createfromdatestring-examples")
                        ->end()
                    ->end()
                    ->scalarNode('rand_end')
                        ->defaultValue(0)
                        ->validate()
                            ->ifTrue(function($v) { return !$this->intervalStringIsValid($v); })
                            ->thenInvalid("Invalid DateInterval string detected: %s. Check http://php.net/manual/en/dateinterval.createfromdatestring.php#refsect1-dateinterval.createfromdatestring-examples")
                        ->end()
                    ->end()
                ->end()
            ->end()
        ;
    }