https://www.taxtips.ca/smallbusiness/t4-slips-requirements-and-timing.htm
T4 Slips - Requirements and Timing
Employment income is included on a T4 for the year in which it is received, not earned, as per s. 5(1) of the Income Tax Act, which states: "a taxpayer's income for a taxation year from an office or employment is the salary, wages and other remuneration, including gratuities, received by the taxpayer in the year." If payment is made in January 2022 for wages earned in December 2021, this amount will be reported on the 2022 T4, not on the 2021 T4. One exception to this is related to employee trusts, which will not affect most employees. Income from an employee trust is reported on a T4 for the period in which the income was allocated to beneficiaries by the trustee, not for the period in which any payment was made to beneficiaries.
Due date for T4 slips to be filed and given to employees: Last day of February, but if this falls on a Saturday or Sunday, the the due date is the next business day.