Expected behaviour:
When clock goes forward into BST, assignment due date of 11:55 PM should stay the same.

Actual behaviour:
When clock goes forward into BST, assignment due date changes to 12:55 AM the next day - i.e. the due date goes forward an hour.

Steps to reproduce:
1. Set up test server and remove NTP.
2. Set system date to before BST starts
3. Create a course with an assignment due after BST
4. Check due date
5. Change system time to after BST starts
6. Check due date, and find that it has changed.

See these steps demonstrated in this screencast: http://users.ox.ac.uk/~cont0147/BST-GMT_due-dates.ogv [8.6MB]
NB: The "timezone" setting in that video's server is set to "Server's local time".

Similar behaviour occurs for the clock going back in autumn.

Looks like the same as discussed here: http://moodle.org/mod/forum/discuss.php?d=109708