Your only real avenue of escape is to ask Hertz to look up the previous renter's record and find out the return time. If the return time is after the traffic ticket, you got a case. If the return record shows something different, sorry but it's over. I bet dollars to donuts the previous renter already returned the car and someone you don't know got the ticket. Maybe it's an employee driving to the car wash and got a ticket and they decided to transfer the traffic fine to you. I've never been to Calgary, but Vancouver doesn't have gates with cameras at their rental car lot to prove the start rental date and time. Maybe you can get photos of the driver from the ticket and send the GM of the facility those pictures to see if it's an employee, to see if the GM is honest, and to see if the GM likes the employee more than a foreign customer. The corporate HQ is going to think it's you unless the previous renter's return time is after the ticket. The credit is just a customer service gesture. It doesn't necessarily change the start time of the rental.
This is one of those cases where you were in the right, and you'll probably still lose even in court. That rental start time really hurts you. Be prepared for the outcome to be pay up and move on. Sorry.
One reason it's always worth suffering the annoyance of the counter agent to read the details before signing; won't help the OP, but a good reminder of why to do it. I've caught errors several times that likely wouldn't have been an issue but just weren't correct; have also removed add-ons not requested or even verbally declined only moments before the contract was presented. Some agents may be trying to slip one over, but most are probably just in a workflow and miss a detail here and there.
May not have helped in this case, but I also always record a walk around of the vehicle in HD on my phone before leaving the spot it's in and complete and turn in the form for damage noted. Yeah, it takes time but worth the peace of mind to me. Also always get a photo of the dash showing mileage and fuel at turn in, widest angle possible to show it's clearly in the return lane at their location; the date and time are in the photo file's metadata.
Yes, there have been a few occasions where a time crunch has conspired to prevent these things; they've always nagged at my conscience though. Anecdotally, the rental companies lately seem to capitalize on any opportunity a customer gives them to pick pockets with impunity.
OP...hope you're able to sort this.