HighCharts: How to display xAxis between a DateTime range? - ruby-on-rails

Question

I am using a line chart with the 'datetime' xAxis. I have pointStart: 'April 08, 2015 07:00 PM' and tickAmount: 12 for a 12 hour display. The graph shows April 8 on the xAxis, but displays 12am to 12pm instead of the 7am to 7pm.
Here is my configuration:
x_axis_start_time = 'April 08, 2015 07:00 PM -700'.to_datetime
#pv_chart = LazyHighCharts::HighChart.new('graph') do |f|
f.chart({:zoomType =>'x'})
f.options[{
useUTC: false,
scrollbar: { enabled: true }
}]
f.title({ :text=>"PV Module Temperature"})
f.xAxis [
{
type: 'datetime',
pointStart: x_axis_start_time,
pointInterval: 1 * 3600 * 1000,
tickAmount: 12,
tickInterval: 1 * 3600 * 1000,
}
]
f.yAxis [
{
title: {text: "Temperature ( °C )",
margin: 30},
}
]
f.legend(:align => 'center', :verticalAlign => 'bottom', :layout => 'horizontal')
f.series( :name=> 'PV1', :data=> #avg_pv1, color: '#FF6138', \
pointStart: x_axis_start_time, pointInterval: 60 )
f.series( :name=> 'PV2', :data=> #avg_pv2, color: '#00A388', \
pointStart: x_axis_start_time, pointInterval: 60 )
end
Result: http://i.stack.imgur.com/DinTq.png
I read on another SO thread that passing in UNIX epoch or milliseconds works, but when I convert the DateTime object to int and * 1000, the graph does not display. How can I start xAxis labels from a particular hour?
EDIT:
start_time = '2015-04-02 07:00:00 -0700'.to_time
end_time = '2015-04-02 19:00:00 -0700'.to_time
f.xAxis [{
type: 'datetime',
pointStart: start_time,
}]
f.series( :name=> 'PV1', :data=> #avg_pv1, color: '#FF6138', \
pointInterval: min_inter * 60, pointStart: start_time, \
tickInterval: 1 * 3600, tickAmount: 12, \
min: start_time, max: end_time)
This configuration works the way I want the points to display, but the xAxis is not using pointStart: start_time. It uses the date though.