Setup Date Select form php - php

Question

I don't have a background in PHP but a recent client requires some updating so i'm doing my best. What I'm trying to accomplish is first update the form to accept the correct date format to covert using the code I created that will through an error if it's less than 24 hours from the present time. The form now:
<p>
<label for="date">Date:<span>*</span></label>
<span>
<input type="text" name="date" id="date" value="<?php $this->value('date'); ?>" size="25" />
</span>
</p>
<p>
<label for="time">Eat Time:<span>*</span></label>
<span>
<input type="text" name="time" id="time" value="<?php $this->value('time'); ?>" size="10" />
</span>
</p>
<p>
<label for="timedes">AM or PM:<span>*</span></label>
<span>
<select name="timedes" id="timedes">
<option value="0" <?php $this->value('timedes', '0', 'selected="selected"'); ?>>Select..</option>
<option value="AM" <?php $this->value('timedes', 'AM', 'selected="selected"'); ?>>AM</option>
<option value="PM" <?php $this->value('timedes', 'PM', 'selected="selected"'); ?>>PM</option>
</select>
</span>
</p>
The code that I will need to modify to validate if it's less than 24 hours or not.
// define variables for date
date_default_timezone_set('America/New_York'); // timezone
$dateInput = '10/11/2015 15:54'; //date they selected string
$cateringDate = (strtotime("$dateInput")); // date selected converted to time
$dateNow = date('m/d/y H:i'); // current date and time string
$currenttime = (strtotime("$dateNow")); // current date and time converted
$dateAllowed = $currenttime + (86400); // current date and time 24 hours added on
$difference = $dateAllowed - $cateringDate; // date selected vs current date + 24hours
if ($cateringDate >= $dateAllowed) {
echo 'good job';
} else {
echo 'try again';
}

Answer 1

There are multiple ways to do this. But here is one using strtotime:
date_default_timezone_set('America/New_York'); // timezone
$dateInput = '10/11/2015 15:54';
if (strtotime('+24 hours') < strtotime($dateInput)) {
echo 'good job';
} else {
echo 'try again';
}
If strtotime did not understand the format it returns false, so:
if (false === strtotime($dateInput)) {
echo 'Sorry, what?';
}

Answer 2

Use the datetime class and take advantage of the diff method, i.e.:
date_default_timezone_set( 'America/New_York' ); // timezone
$date1 = new DateTime( 'NOW' );
$dateInput = '10/10/2015 15:54';
$date2 = DateTime::createFromFormat( 'm/d/Y H:i', $dateInput );
$diff = $date2->diff( $date1 );
if( $diff->format( '%d' ) != 0 ){
echo "good job";
}else{
echo "try again";
}
We check if the difference (in days) between NOW and the $dateInput is different from 0, zero means less than 24h from a future or previous date.