booking functionality with time slots in calendar

First of all we need Calendar and some custom view of day/week/month views.
Copy your tpl.php files from calendar/theme dir into your theme and modify them as you wish.
If you will have problem that template files will not work, look at this: http://drupal.org/node/342614#comment-1148317

In attachment you have example patch for calendar-day.tpl.php theme file (and whole file).
Example with fixed hours slots even there are not any events.
See attachment for demo.
You can change your time range in following line:

    <?php for ($h = 9; $h<=21; $h++): ?> // e.g. FROM 9 to 21

This version example theme for each half an hour (see pic).
Make sure that your grouping settings is set to 'Half hour' in your calendar view.

Lets now do some booking theme.

All you need is to change following lines:

    define('AVAIL_SLOTS', 3); // CHANGE here to set limit if you have one or many slots available in the same time
    define('EVENT_TIME', 3); // for HOW LONG each event should be booked (please put number of half hours, 2 = hour, 3 = hour and half, etc.)
    $slot_booked = t('Booked').'<br>';
    $slot_free = l(t('Book now'),'aa').'<br>'; // CHANGE your link from 'aa' to somewhere where user can book specified event

Now it's configured that there are 3 available slots in one time and each even is 1,5h long (so the real end time of the event will be ignored).

Note that in theme file you can't check permission of specified time slot, if you will not have permission to access specified dates, on different roles there can be different free time slots. But you can easily modify the theme to check if user can create some content and then you can generate message that booking is not available or something.

See screenshot for live example.

P.S. For PHP 4.x you need to change following section into:

    foreach ($booked as $key => $time_left) { // check booking slots
	if (--$booked[$key]<1) { // decrease half an hour and check if it's finished
	    unset($booked[$key]); // if yes, free one slot
	}
    }

Now we need to develop relation between Booking link and creation datetime field.
Make some changes to the above theme:

    $slot_booked = t('Already booked');
    $slot_free = t('Book my event');
    $module_link = 'node/add/my_content_type'; // Change this to pass datetime values to the specified content type

And change content generation to followed:

    /*
     * Set content
     */
    for ($slot=0; $slot<(AVAIL_SLOTS); $slot++) { // now check which slot is...
	if (array_key_exists($slot,$booked) && $booked[$slot]>0) { // ...booked
	    $content .= "<div class='slot_booked'>$slot_booked</div>";
	} else {
	    $link = l($slot_free, $module_link.'/'.$rows['date'].' '.$hh);
	    $content .= "<div class='slot_free'>$link</div>"; // ...and which is free
	}
    }

Now when you click, you will be redirected to page: node/add/my_content_type/2008-12-08 17:20:00

In the form_alter() create something like:

function party_form_alter(&$form, $form_state, $form_id) {
    switch ($form_id) {
	case 'my_content_type_node_form':
	    $fieldname = 'field_my_field';
	    $form[$fieldname][0]['#default_value']['value'] = _get_last_path_arg();
	    $form[$fieldname][0]['#default_value']['value2'] = party_add_time(_get_last_path_arg(),array('hour' => 1, 'minute' => 30)); // here change your period how long even will take time
	    // here inject javascript to hide your datetime field, like: $('#edit-field-my-datetime-0-value-wrapper')[0].parentNode.parentNode.setAttribute('style','display:none'); // hide dates
	break;
    }
}

Above function you will find in attachment.

So basically:
user choosing Booking slot from day view (you can modify your week and month themes as well), then it's going to custom booking form with filled datetime fields and that's it.

Note: of course you can make some your own validation to make sure that user will not book some date when he is not allow to do that.

UPDATE:
After some changes into Drupal website I can't edit my attachment files, so there is small patch for party_add_time() function:
Replace following lines:

    $new_datetime = gmdate('Y-m-d H:i:s',mktime($arr['hour'], $arr['minute'], $arr['second'], $arr['month'], $arr['mday'], $arr['year']));
    return is_array($datetime) ? party_convert_date_to_arr($new_datetime) : $new_datetime;

In attachment you will find my first version of module.
I can't promise that it will work at it should, but you can fully customize it (read README.txt)
Definitely it will save some time for people who tried make similar thing.
If somebody is interested with development of this module, I'm interested as well.

I agree.
I think the first thing is good to implement in Calendar module some settings that will make grid of hours even there are no any events in those slots (like static slots). Option located close to hour and half-hour settings.

redndahead:
you can attach some patches, in my free time I'll create drupal project for this.
At the moment there are some things hardcoded into the code (and I couldn't find time to do that), so some settings have to be moved into the settings page.
In the first time I didn't know that lot of the people will be interested with this functionality.

Ok, anyway, I've created new project:
http://drupal.org/project/booking_timeslots

There are pending issues TODO:
http://drupal.org/project/issues/booking_timeslots

If somebody is interested in development of this module, you're welcome.

dragondad: Please create new issue for that.
booking_timeslots is done

There is Settings Page so it's configurable.

#19: #475318: Fields category does not get populated