I have a similar issue with birthday entries in my Office365 calendar. Some of them are shifted one hour, which could be the UTC offset - I am in Germany.
On the other hand, some are correct. I think, it might be a correlation with the starting date of the series. Older persons are wrong, younger are correct, but I did not find time to check every single date.
Regards, Heiko
PS Anyway I am very happy with AquaMail. I am using the payed version since years.