At risk of making a fool of myself, I think I have spotted it as a bug in https://groups.io/js/displaytime-0.2.6.js

function DisplayEventTime(t, tzstr, dateFormat, timeFormat) {
    if (t == 0) {
        return '';
    }
    if (typeof tzstr === 'undefined' || tzstr == '') {
        tzstr = '';
        momentDay = moment(t);
    } else {
        momentDay = moment(t).tz(tzstr);
<snip>

should, I suspect, have that last line before the snip as                  momentDay = moment.tz(t,tzstr)
- that's what the matching line looks like in the other, similar functions in that file.