The sixty-second answer
April 30 of the following year for most individuals, extended to June 15 if you carried on a business, and to your spouse or common-law partner as well. The late-filing penalty under subsection 162(1) is 5% of the balance that was unpaid at the deadline, plus 1% a month for up to 12 months, so 17% at the cap. If you have been late in any of the three preceding years and the CRA has demanded a return, subsection 162(2) doubles it to 10% plus 2% a month for up to 20 months, so 50%.
Two dates that are usually the same and sometimes are not
An individual has a filing date and a payment date, and the fact that they coincide for most people hides the fact that they are set by different provisions.
The filing date is in section 150. Subparagraph 150(1)(d)(i) requires the return on or before the following April 30 [1].
The payment date is the balance-due day, defined in subsection 248(1). For an individual in the ordinary case, paragraph (c) of that definition also sets April 30 in the following taxation year [3].
Both April 30, so no difficulty. The separation only becomes visible when one of them moves and the other does not, which is exactly what happens to anyone with business income. Clause 150(1)(d)(ii)(A) extends the filing date to June 15 for an individual who carried on a business in the year [1]. The balance-due day definition contains no matching extension. That case is worth its own treatment, and has one.
Two extensions worth knowing
The June 15 extension carries a carve-out. It does not apply where the business expenditures were primarily the cost or capital cost of tax shelter investments [1]. A business existing mainly to generate tax shelter deductions does not buy the extra six weeks.
The extension also travels. 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 covered by clause (ii)(A) [1]. Spouses of self-employed people get the later filing date whether or not they have business income themselves.
There is a third case for bereavement. Subparagraph 150(1)(d)(iii) sets the return of a surviving cohabiting spouse or common-law partner at the later of the ordinary date and six months after the day of death [1], and paragraph (b) of the balance-due day definition makes a parallel substitution for an individual who died after October in the year and before May in the following one [3].
How the penalty is actually calculated
Subsection 162(1) has two components, and both are measured against the same base [2].
The base is the tax payable for the year that was unpaid when the return was required to be filed. Not the total tax. Not the amount still outstanding today. The amount unpaid at the deadline.
On that base the penalty is 5%, plus 1% for each complete month the return remains outstanding, not exceeding 12 [2].
Two words in that carry weight. "Complete" means partial months do not count, so a return filed on day 40 accrues one month, not two. And "not exceeding 12" caps the monthly component at 12%, which brings the maximum under subsection 162(1) to 17% of the unpaid balance.
One technical point: subsection 162(11) directs that tax payable be determined before taking into consideration the specified future tax consequences for the year [2]. A loss carried back later does not retroactively shrink the penalty base.
The repeat penalty is where it becomes serious
Subsection 162(2) is a genuinely different order of magnitude, and it has conditions that must all be met [2].
It applies to a person who fails to file as required, to whom a demand for a return for the year has been sent under subsection 150(2), and who was liable to a penalty under subsection 162(1) or (2) in respect of a return of income for any of the three preceding taxation years [2].
Where those conditions are satisfied, the base penalty is 10% of the unpaid tax, plus 2% for each complete month outstanding, not exceeding 20 months [2]. At the cap that is 50% of the balance that was unpaid at the deadline.
The demand requirement is the practical gate. The doubled penalty is not automatic on a second late filing; it requires the CRA to have sent a demand under subsection 150(2). But once a pattern exists, that demand is not a remote possibility.
The counterintuitive part: file anyway
Because both penalties are percentages of tax that was unpaid at the deadline, a return filed late with a nil or credit balance produces a nil penalty under the section 162 arithmetic. The percentage has nothing to work on.
The far more common situation is the opposite, and it produces advice that people resist. If you cannot pay, file on time regardless. The late-filing penalty is driven by the return being outstanding, and it is the expensive component. Interest on an unpaid balance runs separately and is a different, generally smaller, problem.
Put concretely: a $10,000 balance filed on time and paid late accrues interest. The same balance with the return filed twelve months late accrues $1,700 in penalty before interest. On a repeat, $5,000.
Why returns are late, in practice
Very few people decide to file late. The return is late because assembling it has not started, and assembling it has not started because the underlying material is not in a state where starting is possible.
That is a records problem with a deadline attached, and it recurs annually for the same reason each time. The slips are somewhere, the receipts are somewhere else, and the gap between "I should do this" and "I can begin" is measured in evenings nobody has.
MapleTax is built on the opposite assumption: that the return should be assembled from records that already exist in usable form. Receipts captured in MapleExpense through the year and invoices raised in MapleInvoice are already categorised when the filing period arrives, so the T1 starts from populated figures rather than from a shoebox. It does not make the deadline later. It makes starting cheap enough that the deadline stops being the binding constraint.