Subscriptions are generally paid in advance. If the billing period of subscriptions changes, e.g. because a subscription is changed immediately, the already paid but unused period of the previous subscription is automatically credited to the billing statements.
How this is calculated in detail is shown in this example
A subscription is changed on 01.02.2023 17:31h
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Old billing period: 20.01.2023 00:00 - 20.02.2023 00:00
New billing period: 01.02.2023 17:31 - 01.03.2023 17:31
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remaining Days in old billing period: 31,00 days
Price per old billing period: 99,00 €
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Price per day in old billing period: 3,19 €
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"Unused" and paid time of the old billing period (01.02.2023 17:31 - 20.02.2023 00:00): 18,270139 days
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Credit for "Unused" and paid time in the old billing period: 58,3466 €