Subscription expired date issue when change from monthly to yearly payment

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Since my subscription fee was monthly paid on December 7, 2023, and the expiration date is January 7, 2024.

When I changed the payment from monthly to yearly on December 17, 2023, my expiration date was updated to December 16, 2024.

I thought it should be till January 6, 2025. cloud you advise why there is no membership from December 17, 2024, to January 6, 2025? Please help to clarify this subscription date issue.