Prompt Payment Interest Tracker

Overview & Role#

The Prompt Payment Interest Tracker (prompt-payment-interest, CLIN 0001: Pricing & cost accounting) is a standalone, MIT-licensed static browser utility generated by Moldloft using the deterministic calculator stack template. It determines the statutory payment due date for a federal invoice, counts late days, selects the applicable Treasury semi-annual interest rate from a bundled rate table, and computes simple or compounded late-payment interest.

The tool runs 100% in browser memory with zero backend services, zero runtime AI, and zero network calls after load. It also surfaces statutory deadlines for agency defect notices (7 days) and contractor written demands for unpaid interest penalties (40 days).

Governing Regulations & Sources#

Inputs, Calculation Logic & Outputs#

Inputs

Calculation Logic

1. Effective Acceptance Date (D_acc_eff):
   • Use actual D_acc if provided; otherwise D_del + 7 days (deemed acceptance).
2. Adjusted Payment Window (W_pay):
   • penalty_days = max(0, defect_notice_delay - 7)
   • W_pay = max(0, contract_payment_period - penalty_days)
3. Payment Due Date (D_due):
   • D_due = max(D_inv, D_acc_eff) + W_pay days
4. Applicable Treasury Rate (R):
   • Look up the semi-annual rate in effect on D_due + 1 day (rate locks for the entire late period per 5 CFR 1315.10(a)(2)).
5. Days Late (N):
   • If D_pay <= D_due or dispute_or_financing == true: N = 0, interest = $0.00.
   • Otherwise: N = min(days_between(D_due, D_pay), 365) (accrual stops at 1 year per 5 CFR 1315.10(b)(1)).
6. Interest Math (360-day year per 5 CFR 1315.17):
   • If N <= 30 (simple interest):
     • interest = round_to_cent(P * (R / 100) * (N / 360))
   • If N > 30 (compounded every 30-day period):
     • Initialize principal = P, total_interest = 0, remaining = N.
     • While remaining > 0:
       • chunk = min(30, remaining)
       • period_interest = round_to_cent(principal * (R / 100) * (chunk / 360))
       • total_interest += period_interest
       • principal += period_interest
       • remaining -= chunk
7. De Minimis & Additional Penalty Rules:
   • If total_interest < 1.00: payable_interest = $0.00 (5 CFR 1315.10(c)).
   • Calculate 40-day demand deadline (D_pay + 40 days) and additional penalty (clamp(payable_interest, 25.00, 5000.00) if unpaid 10 days after D_pay).

Outputs

• Computed Dates: Effective Acceptance Date, Due Date, Days Late, and 40-Day Demand Deadline.
• Period-by-Period Interest Table: Shows starting principal, days in period, and accrued interest for each 30-day (or partial) block.
• Total Interest Owed and Additional Penalty Eligibility (with $1.00 de minimis and $25–$5,000 penalty bounds).
• Rate Verification Notice: Displays the effective date range of the bundled Treasury rate table and reminds users to verify against fiscal.treasury.gov/prompt-payment/.

Clean-Room & 5-Condition QA Requirements#

1. Offline Execution: Ships the historical and current Treasury semi-annual rate table as a bundled JSON asset inside the repository; operates completely offline.
2. Strict Content Security Policy: Enforces default-src 'self'; connect-src 'self'; form-action 'none' with zero remote API calls.
3. Zero Tracking or Accounts: Zero sign-in walls, analytics, cookies, or beacons. Any optional localStorage invoice log must be disclosed with a Clear Data button.
4. Worked-Example Assertions & Synthetic Fixtures: Automated tests assert both simple (<= 30 days) and compounded (> 30 days) worked examples, < $1.00 de minimis suppression, 1-year (365-day) accrual cap, 7-day deemed acceptance, and late defect-notice window reduction. Real contractor invoices and CUI are strictly forbidden.
5. Citations, Scope Limits & MIT License: Cites 31 U.S.C. Chapter 39, 5 CFR Part 1315, and FAR 52.232-25; explicitly states exclusions (prime-to-subcontractor payments, OTAs, grants, Contract Disputes Act claims, and disputed/financing payments); includes a non-advice disclaimer and an MIT LICENSE file.

Worked Example & Verification#

Fixture 1: Simple Interest (<= 30 Days Late, from Invoices and late payment interest)

• Inputs: Invoice $20,000.00, Due Date 2026-09-01, Payment Date 2026-09-21 (20 days late), Treasury Rate 4.75%.
• Expected Output:
  • 20,000.00 × 0.0475 × (20 / 360) = $52.78

Fixture 2: Compounded Interest (> 30 Days Late, from Invoices and late payment interest)

• Inputs: Invoice $20,000.00, 45 days late, Treasury Rate 4.75%.
• Expected Output:
  • Period 1 (30 days on $20,000.00): 20,000.00 × 0.0475 × (30 / 360) = $79.17
  • Period 2 (15 days on $20,079.17): 20,079.17 × 0.0475 × (15 / 360) = $39.74
  • Total Interest Owed: $79.17 + $39.74 = $118.91
  • Additional Penalty Range (if unpaid after 10 days & demanded within 40 days): $118.91 (within $25.00–$5,000.00).

Boundary Fixtures

• De Minimis (< $1.00): Invoice $500.00, 10 days late at 4.75% → raw interest $0.66 → Payable interest $0.00 (under $1.00 statutory minimum).
• On-Time Payment (0 days late): D_pay <= D_due → 0 days late, $0.00 interest.

Related pages#

• Invoices and late payment interest
• Check a tool’s answer
• Where the rules live
• The pipeline and its ten workers