Payback Period Calculator implementation

Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

Record owner

Guth Instruments

Lifecycle

verified-clean

Evidence ID

t4-164

Effect

Computation or source lookup only

Input and validation requirements

The input discount_rate = -1 makes every discounted cash-flow denominator (1 + discount_rate)^t equal zero, so the discounted result is undefined. The instrument specification does not state that this invalid rate is rejected. A second robustness risk is the undocumented sign convention for initial cost and cash flows, which can change the meaning of the result.

Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

{
  "inputs": {
    "mode": "simple",
    "initial_cost": 10000,
    "cash_flow_series": [
      2500,
      3500,
      4000,
      3000
    ],
    "cash_flow_period": "annual"
  },
  "expected_outputs": {
    "payback_period_years": 3,
    "payback_period_months": 36,
    "breakeven_point_years": 3
  },
  "working": "Start cumulative cash flow = -10000. After year 1: -10000 + 2500 = -7500. After year 2: -7500 + 3500 = -4000. After year 3: -4000 + 4000 = 0. Therefore payback occurs exactly at 3 years = 36 months; the chart breakeven point is year 3."
}

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche4/tranche4-results.json