code: func (f FrontMatterHandler) HandleDates(d *FrontMatterDescriptor) error {
  	if d.Dates == nil {
  		panic("missing dates")
  	}
  
  	if f.dateHandler == nil {
  		panic("missing date handler")
  	}
  
  	if _, err := f.dateHandler(d); err != nil {
  		return err
  	}
  
  	if _, err := f.lastModHandler(d); err != nil {
  		return err
  	}
  
  	if _, err := f.publishDateHandler(d); err != nil {
  		return err
  	}
  
  	if _, err := f.expiryDateHandler(d); err != nil {
  		return err
  	}
  
  	return nil
  }
docs: // HandleDates updates all the dates given the current configuration and the
  // supplied front matter params. Note that this requires all lower-case keys
  // in the params map.