SEAT BILLING · DAILY PRORATION
Seat proration: charge the remaining period
Add five seats and remove three during a 31-day October, and compare each proration rule with the contract’s daily amount.
Your experiment
acme pays $30 per seat per month, starts October with 10 seats, adds 5 on 11 October and removes 3 on 21 October. The contract prorates daily over the actual 31 days and counts the change day. Start with a fixed 30-day denominator, then match the contract and break it one rule at a time.
Play replays the computed run over sixty inspection seconds; Next event steps, Restart rewinds, Speed changes the pace and Inspection time scrubs. Any input change recomputes the run and rewinds it without autoplay.
Event trace
Computed data
Final metrics
Fictional plan and prices; amounts are rounded to cents only on display and in the final comparison. The contract amount is seats × $30 × remaining days ÷ 31 with the change day counted. Taxes, mid-cycle invoices, minimums and payment timing are out of scope. No billing system is called.
October has 31 days
acme pays $30 per seat per month and starts October with 10 seats, $300. It adds 5 seats on 11 October and removes 3 on 21 October. The contract prorates daily over the actual month and counts the change day: the addition costs 5 × $30 × 21 ÷ 31 = $101.61 and the removal credits 3 × $30 × 11 ÷ 31 = $31.94, for an invoice of $369.68. The opening rule divides by a fixed 30 days instead: $105.00 for the addition and a $33.00 credit, $372.00 in total, OVERCHARGED by $2.32.
Match the contract, then break it
Set Days in the denominator to Actual month length (31) and the invoice equals the contract, $369.68: MATCHES CONTRACT. Now set Change day to Not counted: the addition is charged for 20 days and the removal credited for 10, giving $367.74, undercharged by $1.94. With the 30-day denominator and the change day not counted the two errors partly cancel at $370.00. Off-by-one day counting and the wrong denominator are separate defects that can hide each other.
Credits are part of proration
Set Proration policy to Prorate additions, no credit for removals with the actual denominator: the three removed seats stay billed for the rest of the month, $401.61, overcharged by $31.93. Full month for additions, no credit charges the five new seats a whole $150 as well: $450.00. Clear Remove three seats on 21 October and the correct invoice is $401.61, which the daily rule matches on 31 days but overcharges by $3.39 on 30.
The 30-day error changes sign
Day the five seats are added moves the addition. On 2 October a 30-day denominator charges $150.00 for the addition against a contract $145.16 for 30 of 31 days, and with the credit the invoice is $417.00 against $413.23. On 31 October it charges one day as $5.00 against $4.84, but the bigger over-credit on the removal wins and the invoice is $272.00 against $272.90: UNDERCHARGED. A fixed denominator is not a consistent bias in the customer’s favour or yours.
Reading the run and its limits
Play, Next event, Restart, Speed and Inspection time step through the base charge, the addition, the removal and the invoice across sixty seconds; the table shows each charge beside the contract amount. The plan has one price, one month and whole-day changes; taxes, minimum terms, mid-cycle invoices, refunds and payment timing are left out. The arithmetic is exact and rounded only for display and the final comparison.