Which time zone are my billing months based on?

Your own — the one set in Settings → Basic → Timezone. Komashi billing cycles and partner payout calculations all use it, so your billing month begins on the 1st day at 00:00 in that time zone, not in UTC and not in your browser's time zone.

If you change it, the new time zone applies to future billing and payout calculations only. Calculations that already ran keep the time zone they were recorded with, so past invoices and payout statements are not recalculated.

See Basic Settings for how to change it, and Provider Fees for how the billing month affects your fee.