Why the timing assumption matters
A SIP instalment is debited at the start of the month, so it earns growth for that whole month. In financial terms this is an annuity-due, and the formula is FV = P × ((1+i)ⁿ − 1) ÷ i × (1+i).
Calculators that use the end-of-month form drop that final (1+i) and understate the result every time — by a small amount monthly, but a meaningful one over twenty years. This tool uses the annuity-due form.