Ensure timezone consistency

This ensures `date` filters have a default timezone applied in case the time value does not explicitly specify one. This is something we've done on our internal filters, but is missing in this repo.

It feels a bit odd to do this, because, in some cases, users may be *required* to adjust the time (e.g. `syslog`) since the protocol may not include it and the users are not actually within UTC. However, I think they can/should deal with that in a later, user-specific filter which adjusts the time by `x` number of hours. I think it's more important for our tests to pass consistently across all time zones.