6 answers
It definitely sounds like a Daylight Savings Time issue. Either that, or the calendar just gets spooky around Holloween. Is your PC timezone the same as your user's timezone? The Org is set to DST as is my and the user's machines. It's definitely the issue though, thank you Amnon. Exactly! Now, depending on how they implemented things, it could be to do with a time zone desynchronisation between your local machine and the user record. Say your user is set to a USA timezone where DST already happened, your local machine will essentially be sending one hour backwards when you click the date. So the first thing I'd check is the timezone set on your user. Interesting, with you now. So the 29th, 30th and 31st are working because they are after daylight savings has finished. That's because the buttons use dates, and not date-times. A date is essentially like dd/mm/yy 00:00:00, which means a one hour difference ruins the whole thing.
As someone who's built a few calendar applications on Salesforce, this has been the bane of my existence. Could be that the calendar component is struggling with it, as well.I thought along the same lines but the difference is too big. It's now after 4pm in the UK so there isn't a timezone that is still on yesterday. All users, including me, are in the same timezone.
