Follow the deposits, not just the total
Consider twelve deposits of $100 at a hypothetical 1% monthly rate. The amount saved is $1,200 in both timing cases. With end-of-month deposits, the first payment has eleven months to grow before the final measurement and the last has none. With beginning-of-month deposits, those growth intervals become twelve months and one month.
The resulting ordinary annuity is approximately $1,268.250301320. Moving every deposit one period earlier multiplies that amount by 1.01, giving approximately $1,280.932804333. Rounded to cents, the timing difference is $12.68.
Read the result in context
| Timing | Deposits | Final value | Growth |
|---|---|---|---|
| End of month | $1,200.00 | $1,268.25 | $68.25 |
| Beginning of month | $1,200.00 | $1,280.93 | $80.93 |
The higher figure is not evidence that more money was deposited. It comes entirely from an additional compounding period for each payment. At zero interest, both values become exactly $1,200. Under a negative rate, receiving more time in the model can instead reduce the value of the earlier payments.
Avoid an accidental thirteenth deposit
Changing the start date should not silently change the payment count. An annuity-due model with twelve payments includes time zero and ends its last payment at the beginning of period twelve. It does not add another payment at the final horizon.
A partial final period also needs a rule. This website includes payments only for complete payment periods and gives an incomplete tail period growth without an extra deposit. That rule is stated explicitly because another schedule convention may produce a different, still internally consistent result.
Try the two views
The annuity calculator displays ordinary and due values together. The contributions calculator adds a starting balance, independent compounding frequency and annual contribution increases. Match the payment count and equivalent periodic rate before comparing their outputs.