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In addition to what @john_silver mentioned I would suggest the following.
Here's some things to check.
1. Is the Time and Time zone set correctly for the laptop or desktop that you are using.
2. When you look at the Yahoo Home Page do you see the weather section? If so does it show your correct location? If not can you click on the icon to the right of the location named and set the correct location. You should also be able to remove any other locations.
3. If you are using the New Mail you can set the timezone for your calendar by using these instructions.
Would you be seeing this duplicate when you are looking at your account from a laptop, desktop, phone, or tablet? What browser or email app are you using? If you are using a phone or tablet what kind of device are you using?