MODX Plugin to convert TV date inputs to unix timestamp. By @theboxer

DateToTimestamp.plugin.php

<?php
/*
 * DateToTimestamp Plugin for MODX Revolution
 * @code @theboxer
 * @comments @sepiariver
 *
 * DateToTimestamp is free software; you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free Software
 * Foundation; either version 2 of the License, or (at your option) any later
 * version.
 *
 * DateToTimestamp is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
 * A PARTICULAR PURPOSE. See the GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * DateToTimestamp; if not, write to the Free Software Foundation, Inc., 59 Temple
 * Place, Suite 330, Boston, MA 02111-1307 USA 
 *
 */
 
// Only for use in the MODX mgr
if ($modx->context->get('key') != 'mgr') {
        return;
}

// Enable events 'OnResourceTVFormRender' and 'OnDocFormSave'
$eventName = $modx->event->name;
switch($eventName) {
    // When rendering, convert timestamps back to dates so standard TV Date Inputs can read it
    case 'OnResourceTVFormRender':
        // Must set IDs of TVs on which to operate in the Plugin's Properties
        $tvs = $modx->getOption('tvs', $scriptProperties, '');
        $tvs = explode(',', $tvs);
        $tvs = array_map('trim', $tvs);
        $tvs = array_keys(array_flip($tvs));
        $tvs = array_filter($tvs);
        $tvs = array_flip($tvs);
        
        // John is awesome
        foreach ($categories as $idc => $category) {
            foreach ($category['tvs'] as $idt => $tv) {
                if ($tv->type != 'date' || !isset($tvs[$tv->id])) {
                    continue;
                }

                $timestamp = $categories[$idc]['tvs'][$idt]->processedValue;
                $badValue = $categories[$idc]['tvs'][$idt]->value;

                $categories[$idc]['tvs'][$idt]->set('value', strftime('%Y-%m-%d %H:%M:%S', $timestamp));
                $categories[$idc]['tvs'][$idt]->set('formElement', str_replace($badValue, $categories[$idc]['tvs'][$idt]->value, $categories[$idc]['tvs'][$idt]->formElement));
            }
        }

        break;
    // When saving, convert values to timestamp for sorting and comparing
    case 'OnDocFormSave':
        if (intval($resource->tvs) != 1) {
            break;
        }
        // Must set IDs of TVs on which to operate in the Plugin's Properties
        $tvs = $modx->getOption('tvs', $scriptProperties, '');
        $tvs = explode(',', $tvs);
        $tvs = array_map('trim', $tvs);
        $tvs = array_keys(array_flip($tvs));
        $tvs = array_filter($tvs);

        foreach ($tvs as $tv) {
            if ($resource->get('tv' . $tv) == '') {
                continue;
            }

            $tvObject = $modx->getObject('modTemplateVar', $tv);
            if (!$tvObject || $tvObject->type != 'date' ){
                continue;
            }

            $value = $modx->getObject('modTemplateVarResource', array('tmplvarid' => $tv, 'contentid' => $resource->id));
            if (!$value) {
                continue;
            }

            $value->set('value', strtotime($value->value));
            $value->save();

        }

        break;
}