Magento – Cron jobs are not working

I was created the Cron jobs it is showing on admin panel and I have set the time also, but it is not running.

my code is

system.xml

<?xml version="1.0"?>

<config>
    <tabs>
        <Auto translate="label">
            <label>Auto Replenish</label>
            <sort_order>120</sort_order>
        </Auto>
    </tabs>
    <sections>
        <autoreplenish translate="label" module="autoreplenish">
            <label>Auto Replenish Settings</label>
            <tab>Auto</tab>
            <frontend_type>text</frontend_type>
            <sort_order>1</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>1</show_in_store>
            <groups>
                <general_settings translate="label">
                    <label>Enable Auto Replenish</label>
                    <sort_order>1</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>0</show_in_website>
                    <show_in_store>0</show_in_store>
                    <fields>
                        <enable_disable translate="label, comment">
                            <label>Enable</label>
                            <frontend_type>select</frontend_type>
                            <source_model>adminhtml/system_config_source_yesno</source_model>
                            <sort_order>1</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>0</show_in_website>
                            <show_in_store>0</show_in_store>
                        </enable_disable>
                    </fields>
                </general_settings>
            </groups>
        </autoreplenish>
        <catalog>
            <groups>
                <configurable_cron translate="label">
                    <label>Cron Schedule</label>
                    <sort_order>100</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>0</show_in_website>
                    <show_in_store>0</show_in_store>
                    <fields>
                        <time translate="label">
                            <label>Start Time</label>
                            <frontend_type>time</frontend_type>
                            <sort_order>10</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>0</show_in_website>
                            <show_in_store>0</show_in_store>
                        </time>
                        <frequency translate="label">
                            <label>Frequency</label>
                            <frontend_type>select</frontend_type>
                            <source_model>adminhtml/system_config_source_cron_frequency</source_model>
                            <backend_model>autoreplenish/adminhtml_system_config_backend_autoreplenish_cron</backend_model>
                            <sort_order>20</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>0</show_in_website>
                            <show_in_store>0</show_in_store>
                        </frequency>
                    </fields>
                </configurable_cron>
            </groups>
        </catalog>
    </sections>
</config>

cron.php

<?php

class Easylife_Autoreplenish_Model_Adminhtml_System_Config_Backend_Autoreplenish_Cron extends Mage_Core_Model_Config_Data
{
    const CRON_STRING_PATH = 'crontab/jobs/my_cron/schedule/cron_expr';
    protected function _afterSave()
    {
        $time = $this->getData('groups/configurable_cron/fields/time/value');
        $frequencyDaily = Mage_Adminhtml_Model_System_Config_Source_Cron_Frequency::CRON_DAILY;
        $frequencyWeekly = Mage_Adminhtml_Model_System_Config_Source_Cron_Frequency::CRON_WEEKLY;
        $frequencyMonthly = Mage_Adminhtml_Model_System_Config_Source_Cron_Frequency::CRON_MONTHLY;
        $cronDayOfWeek = date('N');
        Mage::log(date('N'));
        $cronExprArray = array(
            intval($time[1]),                                   # Minute
            intval($time[0]),                                   # Hour
            (frequency == $frequencyMonthly) ? '1' : '*',       # Day of the Month
            '*',                                                # Month of the Year
            (frequency == $frequencyWeekly) ? '1' : '*',        # Day of the Week
        );
        $cronExprString = join(' ', $cronExprArray);
        Mage::log($cronExprArray);
        try {
            Mage::getModel('core/config_data')
                ->load(self::CRON_STRING_PATH, 'path')
                ->setValue($cronExprString)
                ->setPath(self::CRON_STRING_PATH)
                ->save();
        }
        catch (Exception $e) {
            throw new Exception(Mage::helper('cron')->__('Unable to save the cron expression.'));
        }
    }
}

Replenish.php

<?php


class Easylife_Autoreplenish_Model_Replenish
{
    public function run()
    {
        Mage::log("Cron Jobs");
        return true;
    }
}

config.xml

<crontab>

        <jobs>
            <my_cron>
                <run>
                    <model>autoreplenish/replenish::run</model>
                </run>
            </my_cron>

        </jobs>
    </crontab>

I am save the CST time @ 11:38:00 and it's saved on core_config_data as catalog/configurable_cron/time = 11,38,00 & catalog/configurable_cron/frequency = D & crontab/jobs/my_cron/schedule/cron_expr = 38 11 * * *. But it's not saved on cron_schedule.

Can you tell me where I went wrong?

Thanks in advance.