When a child is chosen to sponsor, after the selection process is completed, the sponsor has the option to choose a start date or pick a default date of today and then proceed to the payment page. The issue is when a specific start date is selected it is defaulted to the first day of the following month instead of the date chosen.
Determine if this was migrated to BBIS from BBNC because when migrating from BBNC there are some options that need to be changed. Check the frequency information to ensure it is not hidden to stop donors from picking a start date. Historically with BBNC the start date was also the installment date. When migrating to BBIS the same frequency details may be in place to allow the start date to be selected but this will not apply to the installment dates.