The sixty-second answer
Carrying on a business moves your filing deadline from April 30 to June 15 under clause 150(1)(d)(ii)(A), and your cohabiting spouse or common-law partner gets the same date. But the payment date is the balance-due day, defined separately in subsection 248(1), and for an individual that stays at April 30. The extension is on the paperwork, not the money. Interest on an unpaid balance runs from May 1 even though the return is not yet due.
Two provisions, one of which moves
The reason this catches so many people is that the two dates are set in completely different places in the Act, and nothing in either one points at the other.
The filing deadline lives in section 150. The general rule in subparagraph 150(1)(d)(i) is the following April 30, and clause 150(1)(d)(ii)(A) substitutes the following June 15 for an individual who carried on a business in the year [1].
The payment deadline lives in the definition of "balance-due day" in subsection 248(1). Paragraph (c) of that definition sets April 30 in the following taxation year for an individual in the ordinary case [2].
Read them side by side and the answer is obvious: the definition contains no business extension. Read them six weeks apart, as most people do, and it is not obvious at all.
What the six weeks are actually for
The extension exists because a business return takes longer to prepare than a return made of T4s. A T2125 requires revenue and expenses for the year to be totalled, classified and reconciled, and that work cannot begin the moment the year ends.
What it is not for is deferring payment. The design assumption is that you will estimate what you owe by April 30, pay it, and finalise the return afterwards.
Whether that assumption is reasonable is a fair question. It does assume a business owner can produce a defensible estimate of annual profit six weeks before finishing the calculation of annual profit, which is easy with organised records and close to impossible without them.
The extension travels to a spouse
Clause 150(1)(d)(ii)(B) gives the same June 15 date to a person who was, at any time in the year, a cohabiting spouse or common-law partner of an individual described in clause (ii)(A) [1].
This is not conditional on the spouse having business income. If one partner carried on a business, both returns are due June 15.
The logic is practical: the two returns interact through credits and transfers, so there is no point making one of them due before the figures for the other exist. But note the balance-due day does not move for the spouse either. Both returns are late-filed after June 15; both balances are late after April 30.
The tax shelter exclusion
The extension has a carve-out. It does not apply where the business expenditures were primarily the cost or capital cost of tax shelter investments [1].
The test turns on what the expenditures primarily were, not on the label attached to the activity. An operating business is unaffected. A vehicle whose expenditure is mainly the acquisition of tax shelter investments does not get the additional six weeks.
What each deadline costs when missed
The two dates carry different consequences, and the difference is not small.
Missing April 30 with a balance owing. Interest runs on the unpaid amount. That is a real cost and it compounds, but it is proportionate to the balance and the delay.
Missing June 15. Subsection 162(1) applies: 5% of the tax payable that was unpaid when the return was required to be filed, plus 1% of that amount for each complete month the return is outstanding, to a cap of 12 months [3]. At the cap, 17% of the balance.
The ordering of these matters for anyone who cannot pay. Paying nothing but filing on June 15 costs interest. Paying nothing and filing in December costs interest plus a penalty in the region of ten percent of the balance. Filing remains the cheaper failure by a wide margin.
One more consequence worth noting for the self-employed specifically: the penalty base is the tax unpaid at the filing deadline. For a self-employed filer that deadline is June 15, so a balance paid in May reduces the penalty base even though it was already late for interest purposes.
The form, and where its numbers come from
Self-employment income is reported on Form T2125, Statement of Business or Professional Activities, which the CRA encourages for business or professional income and expenses, and which self-employed commission salespeople may also use [4]. Guide T4002 is the companion publication [4].
The form itself is not difficult. What is difficult is that every line on it is a total of many small transactions, most of which happened months earlier, and none of which were classified at the time.
That is the specific reason the April 30 estimate defeats people. Producing one requires knowing roughly what the year's profit was, and a business that has not been categorising as it goes does not know that in April. It finds out in June, which is six weeks after the money was due.
Making the April estimate cheap
The structural fix is to have the year's figures continuously rather than annually. If receipts are captured and categorised as they arrive, and revenue is recorded as it is invoiced, then a profit figure exists at any point in the year, including on April 30.
That is how MapleTax approaches a T2125. Expenses come from receipts already processed in MapleExpense and coded to the right line; revenue comes from what MapleInvoice has already issued. The June work is review and refinement of a return that is substantially populated, and the April estimate is drawn from the same figures rather than invented alongside them.
The deadline does not move. What changes is whether meeting both of them requires the same work twice.