How do I say if value exists display value otherwise leave well enough alone with simple_form?

Question

I'm using simple_form and I only want the value: @image.start_at.strftime("%m/%d/%Y %I:%M %p") %> to execute if @image.start_at has a value.

<div class="input-group date" id="datetimepicker">
    <%= f.text_field :start_at, class: 'form-control', value: @image.start_at.strftime("%m/%d/%Y %I:%M %p") %>
    <span class="input-group-addon">
      <span class="glyphicon glyphicon-calendar"></span>
    </span>
</div>

For example, this is from my image create/update _form and this works seemingly exactly how I want it to if I already have an image that has a start_at value. However, when I am creating a new image and there is no start_at value yet how can I say don't try to put this value in the input? I'm not interested in having a default value.

Answer 1

<%= f.text_field :start_at, class: 'form-control', value: @image.start_at? ? @image.start_at.strftime("%m/%d/%Y %I:%M %p") : nil %>

based on @luissimo 's answer