Billing cycle day capped at 28

February-only customers used to see odd behaviour because their billing cycle “day” didn’t exist in some months. We cap the cycle anchor at the 28th to make every month consistent.

• Limited the billing cycle day to a maximum of 28 for new and existing subscriptions.
• Enhanced cycle calculation to set the first cycle’s start time to the requested time, removing the special-case logic that shifted late-night signups to the next day.
• Refactored invoice calculation to use the invoice creation date and to update subscription dates from there.