Samsung
Change the Billing Cycle Date auf Samsung
Samsung phones track mobile data use against a billing cycle that starts on the 1st of the month by default. If your carrier bills you on a different date, the on-device counter and your actual invoice drift apart, making the usage warning and data limit unreliable.
This setting only changes the reset date for the local counter. It does not contact your carrier and does not affect your actual data plan or invoice, so entering the correct date is your responsibility.
Where to change it
Open Settings ➔ Connections ➔ Data usage ➔ Billing cycle and data warning ➔ Start billing cycle on. Set the date that matches your carrier's actual billing cycle start and confirm with Set.
At the moment that date arrives each month, the phone resets the counted data usage to 0. If you set a data limit or warning threshold in the same menu, that limit now applies to the correct period instead of the calendar month.
If your carrier changes billing dates or you switch plans, revisit this setting. If usage still looks wrong after correcting the date, check whether roaming or Wi-Fi data is being counted incorrectly, since that is a separate setting.