If I change the next billing date/time, on what basis is the charge/credit per time-unit calculated?

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.