code: func (p *parser) offset() time.Duration {
  	const ctx = "offset"
  
  	p.next()
  	offi := p.expect(ItemDuration, ctx)
  
  	offset, err := parseDuration(offi.val)
  	if err != nil {
  		p.error(err)
  	}
  
  	return offset
  }
docs: // offset parses an offset modifier.
  //
  //		offset <duration>
  //