Found the solution. As it almost always happens, simple "duct tape" solutions tend to be the most workable.
Here's what I did to fix the issue:
1. Opened the config.php inside the 'web' directory in Symfony, and added at the top of the script, right after the opening php tag, the following bit of code:
// Set the default timezone:
2. Opened the app_dev.php inside the 'web' directory in Symfony, and added the same bit of code, also at the top of the script immediately after the opening php tag.
After I saved both files and ran them in the browser, the warning message disappeared and I was able to use Symfony. I assume, I may have to add the date.timezone manually to a few more scripts inside Symfony whenever the warning message pops up. The better solution would have been to figure out why MAMP overrides the default setting, which is something I'm still trying to do, but at least using this Symfony-specific solution I could get the work started.