I've also come across this problem today, both on a client SharePoint site and my own test SharePoint site. Based on some of the responses in this thread, I've tried various combinations of Time Zone and Locale. I'm based in the UK so my sites were correctly set as:
Time Zone: (UTC) - Dublin, London etc.
Locale: English (United Kingdom)
With these settings, calculated dates do not work. However, if I change the Time Zone to (UTC-08:00) Pacific, but keep the Locale as English (United Kingdom) the calculated fields work fine.
There is defintely something strange with SharePoint dates, as I also have an issue on my clients' SharePoint where dates in document libraries are one day behind, even through they show correctly in the document, regardless of changes to the timezone and locale settings. I have also checked the default site settings in the SharePoint Admin, and changed these back and forth, but this does not appear to make any difference.
Hope this helps someone, I am asking my client to raise a ticket with Microsoft to investigate.