Вы захотите использовать опцию: prompt.
из документации Rails:
# Generates a datetime select with a custom prompt. Use :prompt=>true for generic prompts.
datetime_select("post", "written_on", :prompt => {:day => 'Choose day', :month => 'Choose month', :year => 'Choose year'})
datetime_select("post", "written_on", :prompt => {:hour => true}) # generic prompt for hours
datetime_select("post", "written_on", :prompt => true) # generic prompts for all