If you extend the subscription, then the charge is calculated as follows:

Charge **=** (Plan price **÷** Number of days in the plan billing period assuming plan starts from the current date) **x** Number of days extended.

On the other hand, if you reduce the length of the subscription, credits are raised as follows:

Credits **=** (Plan price **÷** Total number of days in the current term of the subscription) **x** Number of days reduced.

**Note**: The above calculation is done to the millisecond if the billing mode set for Chargebee is millisecond-based