Great question — answering as the UphillTech team:
The Canadian eTA is checked automatically/electronically when you attempt to check-in for your flight to Canada. If the details of the passport being used do not match a valid eTA, then you will not be able to check-in. You can attempt to contact Canadian immigration and see if they will change the date for you (there is a 'Contact Us' link on their website), however it's probably just quicker/easier to simply re-apply with the correct details.
Full answer by Doc: https://travel.stackexchange.com/a/193147 (CC BY-SA 4.0).
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