Hi,
I've just installed the calendar plugin for the first time from v1.1 of the 0.5.3 plugin bundle (very nice, thanks!) so possibly there is a reason for this behaviour I'm not aware of, but when editing existing tasks it appears that it is not possible to update the start and end date/time. In order to allow the start/end details for the task to be updated I had to make the following modifications to program/backend/database.php:
# diff /tmp/myrc/trunk/plugins/calendar/program/backend/database.php ./
171c171
< SET ".$this->q('summary')."=?, ".$this->q('description')."=?, ".$this->q('location')."=?, ".$this->q('categories')."=?, ".$this->q('rr')."=?, ".$this->q('recurring')."=?, ".$this->q('expires')."=?, ".$this->q('occurrences')."=?, ".$this->q('byday')."=?, ".$this->q('bymonth')."=?, ".$this->q('bymonthday')."=?, ".$this->q('timestamp')."=?, ".$this->q('notified')."=?
---
> SET ".$this->q('start')."=?, ".$this->q('end')."=?, ".$this->q('summary')."=?, ".$this->q('description')."=?, ".$this->q('location')."=?, ".$this->q('categories')."=?, ".$this->q('rr')."=?, ".$this->q('recurring')."=?, ".$this->q('expires')."=?, ".$this->q('occurrences')."=?, ".$this->q('byday')."=?, ".$this->q('bymonth')."=?, ".$this->q('bymonthday')."=?, ".$this->q('timestamp')."=?, ".$this->q('notified')."=?
173a174,175
> $start,
> $end,
after which things work as I expected them to. Although I'm not using it it looks like the program/backend/caldav.php file would need a similar update to enable editing start and end dates as well.