Source: https://guthlabs.ai/docs/instruments/menstrual-cycle-length-tracker-predictor/implementation/

instrument implementation / implementation-contract

# Menstrual Cycle Length Tracker/Predictor implementation

A source-bound implementation contract for Menstrual Cycle Length Tracker/Predictor; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## 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**: t4-286

**Effect**: Computation or source lookup only

## Input and validation requirements

The specification permits or fails to define behavior for invalid and undefined inputs: a zero-length cycle produces a raw prediction unless explicitly rejected, one start date causes a 0/0 average, and cycle-length variability has no formula. Date-counting convention is also unspecified and can shift the prediction by one day.

## 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": {
    "history_of_period_start_dates": [
      "2025-01-01",
      "2025-01-29",
      "2025-02-26",
      "2025-03-26"
    ]
  },
  "expected_outputs": {
    "predicted_next_start": "2025-04-23",
    "cycle_length_variability": null
  },
  "working": "Differences: 2025-01-29 − 2025-01-01 = 28 days; 2025-02-26 − 2025-01-29 = 28 days; 2025-03-26 − 2025-02-26 = 28 days. Average = (28 + 28 + 28) / 3 = 84 / 3 = 28 days. Next start = 2025-03-26 + 28 days = 2025-04-23. Variability = undefined because no variability formula is provided."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/menstrual-cycle-length-tracker-predictor/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/menstrual-cycle-length-tracker-predictor/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/menstrual-cycle-length-tracker-predictor/qualification/>)

## 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`