Most people know one date: April 30. If you are employed, that is genuinely the whole story — file and pay by April 30 and nothing else applies.
If you are self-employed, there are two dates, they are different, and the gap between them is where the money goes.
Self-employment moves one date, not both
If you or your spouse carried on a business during the year, your filing deadline moves to June 15. Your payment deadline does not: any balance owing is still due April 30.
So the return can be filed in June, but if you pay in June you have been accruing interest since May 1. This catches people every single year, usually in their first or second year of self-employment, and usually because someone told them "you get until June" without saying what for.
Interest is not a penalty, and it is not deductible
CRA charges compound daily interest on an overdue balance at the prescribed rate plus a margin. It is separate from the late-filing penalty, which is 5% of the balance owing plus 1% for each full month the return is late, up to twelve months.
File late and pay late and you get both. File on time but pay late and you get interest only — which is why filing on time matters even when you cannot pay the balance yet.
Instalments are the real fix
If you owed more than $3,000 in net tax in the current year and in either of the two previous years, CRA will generally expect you to pay by quarterly instalments rather than in one lump.
People who pay instalments properly rarely have the April problem at all, because the balance at filing is small. The usual reason instalments go wrong is that they are based on a year that no longer resembles the current one: a strong year paid on last year's figures leaves a large balance, and a weak year overpays and lends CRA money at no interest.
What to do about it
If you are self-employed, put both dates in the calendar, not one. Work out roughly what you will owe before April rather than discovering it in June. And if a balance is going to be large, know that in February — there is a lot that can be done in February and very little that can be done in July.
General information only, current at the date of publication. Tax rules and CRA positions change. This is not advice for your circumstances.