Plan & contract
Rule

A change applies on the 1st

Contracts start, change and end on month boundaries. Part months are not split.

Every contract step hangs on a month boundary. That is why the app never asks you for a free date.

ActionWhen it applies
Contract startson the 1st of a month
Plan changeon the 1st of the next month
Cancellationon the last day of the current month

A part month is not prorated. Whoever holds a contract on any day of a month pays the full base fee for that month and gets the full allowance. For the month you join in, you decide at checkout, see Choose your start date.

Your space can back-date a plan change to the 1st of the current month. You cannot. That too only works while no invoice covers the affected period yet. An issued invoice is never changed afterwards.