Hello, I think that you could give a try to this one: http://labs.perifer.se/timedatepicker/.
Its really simple and It will provide you with a drop down to get the desired time.
For example to generate a list of times in half hour increments, you should do something like:
$(".time").timePicker({
show24Hours: false,
step: 30
});
Where .time is the class of your text_field:
<%= your_time_form.text_field :time, :class => "time", :value => time %>
Of course, then in a helper / controller you will need to manipulate this value to build the desired object (maybe a DateTime instance).