For purposes of this discussion, the day you file a trademark application is referred to as your ‘priority date.’

If this is the first application an applicant files for a specific mark for specific goods/services in a ‘convention country’ (which is pretty much every country) where the applicant has what is referred to as a ‘real and effective industrial or commercial establishment’ (usually the applicant’s home country, but see discussion below), you have six months to file in other countries (for the identical mark, for identical goods) and those subsequent filings receive your original priority date (sometimes referred to as your ‘convention priority’ date, because this process was created by an international treaty known as the Paris Convention.

EXAMPLES:

Newco files for the mark EXAMPLE covering widgets in the U.S. on January 1.
Bad Guy files for EXAMPLE covering widgets in Canada on January 2.
Newco has until July 1 to file in Canada, to receive an earlier priority date than Bad Guy.

Newco files for EXAMPLE covering widgets in the U.S. in January 1.
Newco dawdles until July 2 to think about other countries.
Newco can never use Convention Priority for EXAMPLE covering widgets.
HOWEVER
Newco can file for EXAMPLE for gadgets.
Newco can use convention priority for up to 6 months for EXAMPLE covering gadgets (only).

Newco files for EXAMPLE covering widgets in the U.S.
Newco files for EXAMPLE and logo (or EXAMPLE stylized or EXAMPLE XL) in Canada.
Newco cannot claim convention priority in Canada because the application didn’t cover the identical goods as the underlying application.

Newco files for EXAMPLE covering widgets in the U.S.
Newco files for EXAMPLE covering widgets and gadgets in Canada.
Newco can claim priority for widgets, but not for gadgets, in Canada.

Note re “Real and Effective Industrial or Commercial Establishment’

This is an introductory discussion so I won’t get into a lengthy discussion of ‘real and effective industrial or commercial establishment.’ As a practical matter, most companies file their first application for a mark in their home country or some other country where they do business. There are unusual situations where a company might want to file in an out-of-the-way country, because they want to attract as little attention as possible to their choice of mark or goods (for a variety of reasons). The question arises as to whether the filer has a real and effective industrial or commercial establishment in that jurisdiction. But that is a problem that very few companies ever have.