Don't assume that all timezones have a slash in them

On my computer I use UTC time.

```
/etc/localtime -> /usr/share/zoneinfo/UTC
```

This errors because it ends up returning `zoneinfo/UTC` which is totally wrong.

So instead, we should just strip off the prefix path and assume everything after is correct. Since this is darwin, this should be a safer assumption.

And for reference, there are a good number of timezones listed under `/usr/share/zoneinfo` that aren't nested in a subdirectory.