Proration and Mid-Cycle Changes: The Math That Breaks Every Billing System

Proration is where most billing bugs live. The rules sound simple — charge the difference — until you handle upgrades, downgrades, mid-cycle cancels, seat swaps, and coupons interacting with all four.

# The four models

Immediate with proration: charge the delta now, credit unused time. Standard for upgrades.

Immediate without proration: charge full new price now, discard unused time. Aggressive; only for downgrades to lower tier.

Next cycle: change applies at renewal. Cleanest, worst UX for upgrades ("but I want it now").

Custom period alignment: align mid-cycle change to a fixed anchor (calendar month, contract renewal). Enterprise pattern.

Pick one primary and document the exceptions.

# The formula

Unused-time credit:

credit = (old_price / period_length_in_days) * days_remaining

New-period charge:

charge = (new_price / period_length_in_days) * days_remaining

Net:

proration_amount = charge - credit

# The edge cases that break it

• Leap years: period_length_in_days is 365 or 366. Use actual days in the current billing period, not a constant.
• Timezone boundaries: "days remaining" is undefined across DST. Anchor to UTC.
• Refunded credits: if the original charge was partially refunded, credit is on the net paid, not gross.
• Coupons and discounts: apply after proration, not before. Otherwise upgrades give free money.
• Seat additions in the last hour: most systems charge a full day's proration for adding a seat 30 seconds before renewal. Add a "no-op window" of the last 4 hours.
• Downgrades below current usage: what happens to seats/features already in use? Grandfather until renewal, don't kick users out.

# Coupons and proration together

The interaction rule most systems get wrong:

1. Calculate the base proration on undiscounted prices.
2. Apply percentage discounts to the charge portion, not the credit portion.
3. Fixed-amount discounts stay on the original schedule and don't multiply.

Otherwise a 100%-off coupon on a $100 upgrade nets to a $50 refund, which is not what anyone intended.

# The UX rule

Show the customer the exact math before they confirm. Every upgrade dialog should include:

Prorated charge today:    $47.19
  New plan (10 days):     $80.55
  Credit unused Pro:     -$33.36
Next renewal (Mar 1):     $199.00

Customers accept complex billing when it's explicit. They dispute what surprises them.

# Testing proration

Write tests that cover:

• Upgrade on day 1 of period
• Upgrade on last day of period
• Downgrade mid-period
• Upgrade + downgrade in same period
• Coupon applied before upgrade
• Coupon applied after upgrade
• Refunded prior invoice + upgrade
• Seat change of +1, -1, +10, -10
• Change across DST boundary
• Change across leap-day

If any test is missing, that's the next customer complaint.