Passing [disabled: true] does not seem to disable time_select/3. (I’m using a custom builder function:

def time_builder(form, field, opts \\ []) do
  builder = fn b ->
      assigns = %{}

      ~H"""
        <%= b.(:hour, [options: Keyword.get(opts, :hours, hours), class: Keyword.get(opts, :class), size: 1]) %>
        <%= b.(:minute, [options: Keyword.get(opts, :minutes, Enum.map(0..60, & &1)), class: Keyword.get(opts, :class), size: 1]) %>
      """
    end

    time_select(form, field, [builder: builder] ++ opts)

# implementation example

<%= time_builder(f, :start_time, [disabled: true]) %>

FWIW I’d use time_input if it let me enforce 15 minute increments.