Section 63 of the BC Employment Standards Act (ESA) provides that:
(1) After 3 consecutive months of employment, the employer becomes liable to pay an employee an amount equal to one week's wages as compensation for length of service.
(2) The employer's liability for compensation for length of service increases as follows:
(a) after 12 consecutive months of employment, to an amount equal to 2 weeks' wages;
(b) after 3 consecutive years of employment, to an amount equal to 3 weeks' wages plus one additional week's wages for each additional year of employment, to a maximum of 8 weeks' wages.
(3) The liability is deemed to be discharged if the employee
(a) is given written notice of termination as follows:
(i) one week's notice after 3 consecutive months of employment;
(b) is given a combination of written notice under subsection (3) (a) and money equivalent to the amount the employer is liable to pay, or
(c) terminates the employment, retires from employment, or is dismissed for just cause.
(4) The amount the employer is liable to pay becomes payable on termination of the employment and is calculated by
(a) totalling all the employee's weekly wages, at the regular wage, during the last 8 weeks in which the employee worked normal or average hours of work,
(b) dividing the total by 8, and
(c) multiplying the result by the number of weeks' wages the employer is liable to pay.
(5) For the purpose of determining the termination date under this section, the employment of an employee who is laid off for more than a temporary layoff is deemed to have been terminated at the beginning of the layoff.
(6) If, after 3 consecutive months of employment, an employee gives notice of termination to the employer and the employer terminates the employment during that notice period, the employer is liable to pay the employee an amount equal to the lesser of
(a) an amount in money equal to the wages the employee would have earned for the remainder of the notice period, or
(b) an amount in money equal to the amount the employer is liable to pay on termination.
The probationary period under the ESA is thus "3 consecutive months of employment." It is not specified in terms of days or weeks. Presumably this means calendar months. As defined in section (1) of the ESA:
"work" means the labour or services an employee performs for an employer whether in the employee's residence or elsewhere.
(2) An employee is deemed to be at work while on call at a location designated by the employer unless the designated location is the employee's residence.
It would seem that the period of employment starts on the first day that an employee performs work, and ends when the employee is given notice of termination, unless there has previously been a layoff, in which case it ends at the start of the layoff.
However, the post "Evaluating Probationary Employees" from the Lawson Lundell Labour and Employment Law Blog says:
In a non-union workplace, the standard probationary period is often three months, which is the length of time before the employer will owe the employee one week's notice or pay in lieu of notice under section 63 the BC Employment Standards Act (the “Act”). It is important to note that employers and employees may not contract out of the Act. ...
In a unionized workplace, if a collective agreement outlines a mechanism for ending employment, section 63 of the Act does not apply. This means that the probationary period set out in the collective agreement may exceed three months. Indeed, a collective agreement probationary period could be based on cumulative hours worked rather than linear time.
Also, the First Reference page on "Probationary periods in Canada: Are they legal?" says:
In British Columbia, the Employment Standards Act allows either the employer or the employee to end the employment relationship immediately without any severance or compensation within the first three months of employment. However, in Canada we typically don’t recognize an implied probationary period under contract law. This can create an difficult scenario such as the one found in Cao v. SBLR LLP,  O.J. No. 3328.
In Cao, a young accountant was hired by a tax firm further to an extensive interview process. When offered the job, the accountant entered into an employment agreement with the employer. The contract did state that there was a probationary period, but did not provide any language limiting severance during this time.
The accountant started her work but was let go within her first month on the job. The employer took the position that because she was terminated within the probationary period, she was not entitled to any severance. Ultimately, Ms. Cao filed a wrongful dismissal action before the Ontario Small Claims Court and was eventually awarded $20,000 in damages following trial, or 4 months wages.
So how was this possible? In this case, the employer would have complied with its obligations under employment standards law, but it continued to have a duty to provide reasonable notice of termination in accordance with contract law and common law norms.
The page "Probationary Periods and Terminations in Canada" from KCY at Law says:
In the past, employees had few to no rights when they were on probation. However, things are beginning to change as courts recognize the vulnerability of employees beginning a new job.
Without justification for their termination during the probationary period, an employer may still have to pay termination pay to a probationary employee, especially if this employee has been recruited from another secure job position.
Courts now recognize that employees must be given a fair and reasonable opportunity to demonstrate their capacity and fit for the position for which they were hired as was demonstrated in the case of Cao v. SBLR LLP
Employers generally have the right to terminate an employee during their probationary period without notice or pay in lieu thereof, with or without cause, for any reason that is not discriminatory.
Nevertheless, employers should ensure that your employment contracts for new employees clearly set out the probationary period and the period during which they will be evaluated.
Given all this, and particularly if there was a Union agreement specifying a probationary period different from the default 3 months, it may be essential to consult an employment lawyer to determine if there is a valid claim. Note that claims under the ESA must be filed within 6 months after the end of employment.