A customer upgrades from your ₱5,000/month plan to ₱12,000/month on day 15. You invoice the prorated difference, add SST at 6%, and the number that appears on the ledger is ₱2,310. Three days later, your accountant says it should be ₱2,309.50. You dig through the invoice, find the math is right, then spend two hours tracking the ₱0.50 back to a rounding choice your invoicing software made—and that same software will make it differently next month. Proration is simple in theory: charge for the days used. In practice, it breaks fast. Monthly to annual upgrades, mid-cycle plan downgrades, subscription pauses, and regional sales tax recalculation all trigger proration logic. Most invoicing platforms handle the happy path (clean monthly upgrades) but fail the rest. The result is manual reconciliation work, audit flags, and the slow corruption of your financial records. The three proration traps that break reconciliation Proration sounds like a narrow technical problem. It's actually the fault line between your billing engine and accounting system, and it exposes three recurring failures. 1. Upgrade/downgrade math at arbitrary cycle points Your customer is on a ₱5,000/month plan, billed on the 1st. On day 15, they upgrade to ₱12,000/month. The software must calculate: Days remaining in current cycle: 16 days (15th to 30th, inclusive) Difference per day: (₱12,000 − ₱5,000) ÷ 30 = ₱233.33/day Prorated charge: ₱233.33 × 16 = ₱3,733.28 Apply SST (6%): ₱3,733.28 × 1.06 = ₱3,957.28 But the software has a choice: Does it calculate 30 days per month, or 365 ÷ 12 = 30.417 days? Does it round after tax or before? Does it count the 15th and 30th as full days, or does it use a day-count convention (ACT/360, 30/360)? Most platforms use 30-day months and round after tax. Some use calendar days. A few use year-over-year averages. The difference is usually a few paise, but multiply by 50 billing events per month, and your general ledger is drifting. 2. SST recalculation on partial periods Here's where it gets thorny. Your customer's full-month bill is ₱5,000 + SST. But when you prorate a mid-cycle upgrade, do you: Option A: Calculate SST only on the prorated charge? (SST applies to the upgrade only, not the full month.) Option B: Recalculate the entire month's bill with the new rate and tax everything together? Option C: Apply SST separately to the old plan (days 1–14) and the new plan (days 15–30), then bill the difference? The tax authority in Malaysia (LHDN) expects Option C: tax each period's revenue in isolation, then bill the delta. But most subscription platforms default to Option A because it's simpler to code. The result is silent underpayment of SST, which your accountant catches during reconciliation—or worse, during an audit. 3. Rounding cascades into next-month errors Proration generates fractional amounts. ₱3,733.28 is common. When you round to two decimal places, you have a choice: round down (₱3,733.28), round to nearest (₱3,733), or round up (₱3,734). Most platforms round to nearest. But if your next billing cycle lands on day 31 of a month with 30 days—or if the customer's billing date shifts after a proration—the rounding rule can force a correction invoice or credit on the following cycle. That correction is tax-deductible, but it's a separate line item. If your invoicing software doesn't generate it automatically, your accountant has to manually record it. Over a year, with 50+ customers, manual corrections add up to hours of reconciliation work. Which platforms handle it, which don't Testing the same scenario (₱5,000/month upgraded to ₱12,000/month on day 15, 6% SST) across the leading invoicing platforms: Stripe Billing: Handles prorated upgrades cleanly. Uses calendar days, applies tax after proration. Correct for most use cases, but you must manually calculate SST recalculation if you're following LHDN rules strictly. No correction invoice auto-generation if rounding drifts. Chargebee: Proration logic is configurable: 30-day month, 365-day year, or calendar days. Allows you to choose when to apply tax (before or after proration). Best-in-class flexibility, but requires setup work upfront. Tax recalculation is still manual. FreshBooks: Proration works for simple upgrades. Mid-cycle downgrades often require manual invoice creation. SST handling is template-based, not rule-based, so it doesn't adjust automatically on proration. Rounding is fixed (to nearest paise), no override. Xero: No native proration engine. You invoice the full month, then create a separate credit note and new invoice if the customer changes plans mid-cycle. This is tax-compliant (each period is taxed separately) but labour-intensive. Requires your team to manually calculate the prorated amounts. QuickBooks Online: Proration support is limited to 1099 contractors (partial-month invoicing). Subscription billing requires a third-party app (e.g., Stripe or Chargebee) or manual invoices. No built-in SST recalculation. None of them automatic