Date argument not respecting site's timezone - reproducible

I have a site built largely around the Date and Calendar modules. When I upgraded to Date 6.x-2.8 (from 2.7), my calendar's "Day" view display started showing events from the previous day if their start time was after 18:00. In other words, /calendar/2012-01-30 would show events from between 18:00 on Jan. 29 to 15:59 on Jan. 30.

My calendar view's Day display is overridden from stock configuration, using the Views Accordion style rather than Calendar display style. When I switch it to the Calendar display style, it displays events on the right date regardless of their start time. When I try HTML List, it behaves the same as Views Accordion, with most events displaying on the wrong day.

This behaviour continued when I tried it with Date 6.x-dev and on a vanilla Drupal setup with only Date, Calendar, CCK and Views installed.

My "Event" content type has a CCK Datetime field (with optional "To" field and repeats), set to match site's time zone.

My theory is that in the absence of Calendar module's handling the Day display, the date argument is not being offset from UTC to my site's timezone (America/Montreal). When I change the site's time zone by 2 hours, events get shown a day late if they started at 16:00 or later, instead of 18:00.

Is this by design or is it a bug? If it's by design, is there something I could add via the PHP option for default argument validation (usually set to "current date" with basic validation) that would adjust for the offset? What changed from Date 2.7 to Date 2.8 that would cause this different behaviour? I'm assuming that the issue comes from the Date module, because none of the other involved modules were updated, and rolling back to Date 2.7 fixed the problem.

BTW I have tried setting the start of week day to Monday and ISO-8601 week numbers ... no dice. And the server itself is located in my timezone (running PHP 5.2), with the time set correctly.

Thanks, any help much appreciated.