Refactor how timezones are applied during event pull

[matt-bernhardt]

This refactors how event times are processed during import. The legacy approach (used on both the legacy AWS infrastructure as well as the Pantheon/Lando containers) relied on a WordPress-focused approach. While that worked on our AWS environment, it didn't have the same results in Pantheon/Lando.

As an alternative, this change focuses on a PHP-centric approach to time calculations - which I suspect is working because it doesn't rely on the container itself being set to any particular timezone.

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[JPrevost]

I haven't confirmed this is working, but this looks like a good approach. Approving with the assumption we can tweak it if we find edge cases where it gets weird.