Evernt reminders are showing incorrect time even though the event on the calendar is correct.
Please fix your upgrade processes so that it doesn't almost always mess up the groups event calendar and reminders.
This problem occurs every time yahoo groups is upgraded substantially.
You didn't mention whether you have set your group's reminder time zone correctly. As a moderator of the group (with the privilege to change group settings) or an owner you ought to be able to adjust this yourself. In the Events calendar select "Options" from the Actions menu.
The General options page should have a Time Zone setting. That setting is to control the time zone used for generating reminders. Pick the desired time zone then click the blue Save button. That should do it.