ParBEGIN or END mode?
The payment timing switch says whether each payment happens at the start or the end of its period.
END: an ordinary annuity
Most loans pay at the end of each period: you borrow today and the first payment is due a month later. END is the default. Mortgages, car loans and most savings plans use it.
BEGIN: an annuity due
Rent, leases, insurance premiums and retirement withdrawals are paid at the start of each period. Press gBEG7 and the BEGIN indicator lights up. Press gEND8 to switch back.
With BEGIN, each payment has one period more or less to earn interest. Payments to cover a given amount are a bit smaller, and the value of a stream of payments is a bit larger, by a factor of (1 + i).
A common trap
The setting stays on until you change it. If a mortgage payment looks slightly low, check whether BEGIN is lit.