Cash method: taxed when the retainer hits the bank
Most independent agencies file on the cash method of accounting, and under it the rule is blunt: income is taxed in the year you receive it, regardless of when you perform the work. A client who prepays a $30,000 quarterly retainer on December 20 hands you $30,000 of taxable income in the closing year, even though nearly all of the fulfillment happens in the next one. The expenses that will offset it, contractor payments, ad management hours, tools, land as deductions in the year you pay them, which may be the following year.
The cash method's virtue is that it taxes money you actually have. Its edge case is exactly this year-end mismatch, so agencies with prepaid annual or quarterly retainers should watch December closely. One nuance to respect: constructive receipt. A check delivered and available to you in December is December income even if you deposit it in January; you cannot defer income simply by leaving it uncashed or asking a client to hold a payment you were entitled to.
Accrual agencies and the one-year deferral
Agencies on the accrual method recognize income as it is earned rather than when cash arrives, which lines revenue up with the work. For advance payments like prepaid retainers, though, the tax code does not allow unlimited deferral: under the advance payment rules, a prepayment can generally be deferred only into the year after receipt. A two-year prepaid arrangement cannot ride unrecognized until the work finishes; whatever remains deferred gets picked up in year two.
Accrual also means invoiced-but-unpaid retainers are taxable when earned, so an agency with slow-paying clients can owe tax on money it has not collected. That trade, cleaner matching against cash-flow exposure, is why most agencies without a compelling reason stay on cash. The cash method is broadly available to service businesses under the gross receipts ceiling, a threshold well into the tens of millions and adjusted for inflation, so eligibility is rarely the constraint for an independent agency; check the current figure if you are approaching that scale.
What this means for December
The timing rules turn year-end invoicing into a real planning lever. A cash-method agency having a big year can invoice January 1 instead of December 26 for work starting in January, legitimately moving the income into the new year; there is nothing aggressive about billing when the engagement actually begins. Conversely, in a down year it may be worth collecting outstanding invoices before December 31 to absorb income while the bracket is low.
The mirrored move exists on the expense side: paying January's contractor invoices or converting tools to prepaid annual plans in December accelerates deductions into the high-income year. What the rules do not permit is pretending: backdating, parking earned income, or asking clients to delay payments already due. Time the real events, invoices sent, work started, bills paid, and the timing works for you instead of surprising you.