I would like to know when a Location's offset from UTC is going to change. I see that this information is known by the time package, obviously or it wouldn't be able to account for daylight savings. That way I could find out that for the location "America/New_York" daylight savings begins on Sunday, March 11 2018 at 07:00 UTC.
Is there a way to do this short of making my own copy of the time package that exports the Location properties or writing my own parser for the time zone files?
As you've seen in the source, no, that information is not exposed. But, as you can also see from the source, the raw TZDB used to generate the timezones is included in the Go distribution:
//go:generate env ZONEINFO=$GOROOT/lib/time/zoneinfo.zip go run genzabbrs.go -output zoneinfo_abbrs_windows.go
If you take a look at your $GOROOT/lib/time you'll find that file, which contains all the data used to generate the time zone list, and you can look at $GOROOT/src/time/genzabbrs.go to see how it's used.