A charge is posted when upgrading/downgrading to a new plan, when changing the billing cycle, or when ordering a new site in your account. The charge starts from the order processing date and going for the full billing period, rounded down to the account's Monthly Billing Day.
Ordering new site example:
For a 1-Year plan with a Monthly Billing Day of the 23rd, processed on May 12th, the charge will be from May 12th of this year to April 23rd of next year. It will not charge to May 23rd because that would be a charge of OVER 1 year.
Plan upgrade/downgrade or billing cycle change example:
For a 1-Year plan with 4 months remaining and a Monthly Billing Day of the 7th, processed on July 16th, the charge will be from July 16th of this year to July 7th of next year. The 4 months remaining is credited and applied to the charge and has no further bearing on the new billing period.