Yahoo Calendar is known to have issues displaying events and timezones. This is because Yahoo sometimes sets an incorrect timezone for users, which may cause events to show up at the wrong time. Once you set the correct timezone for your Yahoo account, your events will be displayed at the correct time.
Follow these steps to adjust your Yahoo Calendar timezone settings:
In Yahoo Mail, click the Calendar icon.
Mouse over the Settings icon in the top right corner, and select Calendar Options.
Select your time zone from the Time Zone drop-down menu.
Click Back to Calendar.