create an uneditable character in ruby textfield

I need to create an uneditable semicolon in Rails text field for a date.

For instance, the user can enter the hours and the minutes, while the semicolon already there and is not deletable.

 <%= text_field_tag(:start_time, '', class: "timepicker four", size: 5, 
  maxlength: 5, placeholder: 'HH:MM', value: 
  @booking_item.start_time.strftime('%H') + ":"+ 
  @booking_item.start_time.strftime('%M')) %>

Any solution for this in Ruby?

Thanks