When a user sets the available until date for an activity the timestamp value is set to 23:59:59 using strtotime() before it is saved. This causes the date value to be offset by the difference between the creators timezone and what appears to be the Moodle timezone. When the creator then views the activity, instead of showing the available from and until dates it shows the from date and the available until date + or - the time zone offset, for example:

A user with profile timezone set to Australia/Adelaide where Moodle timezone is set to Australia/Melbourne creates a new activity with restrict dates set to '18 August 2011' until '19 August 2011' will save and display as:
Restricted: 'Available from 18 August 2011 to 19 August 2011, 11:29 PM.'

Attached is a patch with a potential solution that replaces strtotime with the make_timestamp() and date() functions.

Luke