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The only fixed thing of your bill is the date of issuance. The collection date has no set date and was always within 10 days/two weeks of a bill been issued. Following changes to billing by Vodafone they have now set new collection dates based on your original bill issuance date. There is nothing here that was done without your permission or against your contract. The bill issue date for you is still the same as before, it's just the collection date that may have changed.