code: function interval(start, end) {
    if (arguments.length === 1 && typeof start === 'string') {
      return start
    }
    else if (arguments.length === 2) {
      var interval = utils.args(arguments, 0)
  
      return interval.map(function(original) {
        var arg = utils.date(original)
  
        if (!arg) {
          throw new FieldError('Bad date specified: ' + original)
        }
  
        return JSON.stringify(arg).replace(/"/g, '')
      }).join('/')
    }
    else {
      throw new FieldError('Bad arguments')
    }
  }
docs: Generate interval string
  
  @param {string|number|Date} start Start time or interval string
  @param {string|number|Date} [end] End time
  @returns {string} Interval string