moment.js timezones .valueOf() not returning expected values

I have an angular application using highcharts-ng to make a line graph. The y-axis is numbers and the x-axis is datetime's.

I am trying to properly account for Daylight Savings Time changes when converting between the two timezones of "America/New_York" and "Europe/London" using moment.js.

London is currently in BST (as of the time of posting this), so it is +1:00.

tick.TimeStamp
> "2015-04-21T16:06:06.0786392-04:00"

tick.TimeStamp is my "America/New_York" time (currently in EDT). I convert that to London time using...

moment(tick.TimeStamp).tz("Europe/London").format()
> "2015-04-21T21:06:06+01:00"

I need my result in Unix Epoch ticks to plot them for the x-axis in highcharts-ng, so I use...

var d = moment(tick.TimeStamp).tz("Europe/London").format()
moment(d).valueOf()

which yields

1429646766000

The issue is that this tick value result as a datetime is

Tue, 21 Apr 2015 20:06:06 GMT

where it should be

Tue, 21 Apr 2015 21:06:06 GMT

since London is currently in BST +1:00

Am I doing something wrong, or is moment just calculating this incorrectly?

Any help would be greatly appreciated. Thank you!

EDIT: I should mention that my moment-timezones.js is the most recent from their site with all of the timezone info.

from moment.js timezones .valueOf() not returning expected values