GopherJS doesn't have access to the full timezone database, due to space restrictions. See here for details and discussion: #64

Thanks @flimzy,
I initially saw that but dismissed it thinking I was only interested in "local" (which gopherjs sort of supports). Which led me to finding JS's toLocaleString() I thought oh this is probably just a missing options in gopherjs because I could easily do the following in JS:

var event = new Date(Date.UTC(2012, 11, 20, 3, 0, 0));
console.log(event.toLocaleString('default', { timeZoneName: 'short' }));
console.log(event.toLocaleString('default', { timeZoneName: 'long' }));

will output:

12/19/2012, 10:00:00 PM EST
12/19/2012, 10:00:00 PM Eastern Standard Time

But after looking at #64 in detail, I think my issue is related. :-(
Maybe I'll use goment (an pure golang implementation of moment.js mentioned in #64) instead.