Changes have been made in Git (master):

commit 2d71e345f61047f3c1ecd1e9ea1fee6f3df82479
Author: Jan Schneider <jan@horde.org>
Date:   Fri May 24 17:02:35 2013 +0200

     [jan] Fix editing of events per CalDAV if client sends 
LAST-MODIFIED attribute (Bug #12244).

  kronolith/docs/CHANGES        |    2 ++
  kronolith/lib/Application.php |    2 +-
  kronolith/package.xml         |    4 ++--
  3 files changed, 5 insertions(+), 3 deletions(-)

http://git.horde.org/horde-git/-/commit/2d71e345f61047f3c1ecd1e9ea1fee6f3df82479

I can also confirm that the workaround fixes the update-event problem, 
but it looks like Lightning/CalDAV has some other flaws. The updated 
event is shown in Kronolith fine, but Thunderbird need a restart to 
actually show the changes done :-(

This code is hit, when one modifies an event.

The problem is actually, that $content->getAttribute('LAST-MODIFIED') 
returns an integer (seconds since epoch), hence, 
$content->getAttribute('LAST-MODIFIED')->before($modified) fires that 
PHP error.

$modified is a string in ISO-format.

My workaround is to cast the integer into ISO-format for comparison, 
if getAttribute('LAST-MODIFIED') returns an integer only.

I posted my upgrade path on 15 May 2013 15:57:52 to the list with 
subject "CalDAV change event fails with PHP error", maybe something 
got screwed up then.

For me the problem exists with all events created with Tunderbird and 
with Kronolith. It is simply not possible to alter an existing event 
with Tunderbird/Lightning in the CalDAV calendar. Creating/Deleting 
works fine.

Cannot reproduce. Is this with all events? Only certain? Only old, 
only new events?

Hi!

I have the same problem when modifying entries with CalDAV Sync App on 
Android 4.1.2 and the workaround is successful.

I use this workaround:

                 try {
                    if(!empty($modified)) {
                        $new_data_time = $content->getAttribute('LAST-MODIFIED');
                        if(!empty($new_data_time)) {
                            //Horde::logMessage('SKA: last-modified: '.$new_data_time. " 
".date("Y-m-d H:i:s", $new_data_time), 'NOTICE');
                            //Horde::logMessage('SKA: modified: '.$modified, 'NOTICE');
                            if(is_object($new_data_time) && $new_data_time->before($modified)
                             || !is_object($new_data_time) && date("Y-m-d H:i:s", 
$new_data_time) < $modified) {
                                /* LAST-MODIFIED timestamp of existing entry is newer:
                                 * don't replace it. */
                                //Horde::logMessage('SKA: modified entry is newer skip update', 'NOTICE');
                                continue;
                            }
                        }
                    }
                 } catch (Horde_Icalendar_Exception $e) {
                 }

Editing a event created with Thunderbird/Lightning (latest on Windows) 
in a Kronolith calender subscribed with CalDAV result in 
"MODIFICATION_FAILED" error.  The server log shows the following:

2013-05-10 10:12:38: (mod_fastcgi.c.2701) FastCGI-stderr: PHP message: 
PHP Fatal error:  Call to a member function before() on a non-object 
in /var/www/horde/kronolith/lib/Application.php on line 822