I have subscribed to My Reminder Calendar in Google Calendar. I have noticed that added reminders are shown 1 day before the actual offer start date, even though I always select "on the start date". For example, this deal has an all-day start date in my calendar on 30/10, even though the start date is 31/10.
There's inconsistent behaviour with the Add to Google Calendar and Export iCalendar functionalities as well, better demonstrated with this deal:
Add to Google Calendar: correct start date, incorrect end date
Export iCalendar: correct start date, incorrect end date
However, Export iCalendar does get the all-day deals correct (e.g. above Halloween one) – see here.
Somewhere your time zones are set differently on one of your services, accounts, or devices