Xiaomi
Set the Monthly Data Usage Reset Date auf Xiaomi
The data usage counter on Xiaomi phones tracks consumption based on a fixed reset day, not your carrier billing cycle. If these two dates do not match, the remaining allowance shown in the phone never lines up with what your provider actually counts. This gets noticeable when you approach a monthly limit and the warning triggers too early or too late.
The setting responsible for this is the usage reset date inside the Data plan section. It only controls the local counter on the device, it does not communicate with your carrier and does not change your actual billing date.
To align it, go to Security ➔ Settings ➔ Data usage ➔ Data plan ➔ Usage reset date. Pick the day your carrier starts a new billing cycle and confirm with OK. The counter resets to zero on that day every month going forward.
If your carrier offers an app or SMS service showing exact remaining data, trust that over the phone counter, since background processes and Wi-Fi traffic can be counted differently depending on system settings. Use the reset date mainly as a rough local warning, not as a replacement for your provider's own usage tracking.