This pulls comic starting from a user-defined day and getting a user-defined number of days after that. See early comments for licensing information.

getter.js

// MIT License Copyright (c) 2019 Benjamin Cooper
// I take no responsibility for your use of this code.

// you'll have to install these modules with npm/yarn
const download = require('image-downloader');
const fetch = require('node-fetch');
const cheerio = require('cheerio');

Date.prototype.addDays = function(days) {
  var date = new Date(this.valueOf());
  date.setDate(date.getDate() + days);
  return date;
};

const getImageUrl = async pageUrl => {
  const $ = cheerio.load(await (await fetch(pageUrl)).text());
  return $('img.img-comic')[0].attribs.src;
};

const getImages = async (start, numDays) => {
  const url = 'https://dilbert.com/strip/';
  const date = new Date(start);

  for (let i = 0; i < numDays; i++) {
    const datePath = `${date.getFullYear()}-${numberToDouble(date.getMonth() + 1)}-${numberToDouble(
      date.getDate(),
    )}`;
    console.log(datePath);
    const imageUrl = `https://${(await getImageUrl(`${url}${datePath}`)).slice(2)}.gif`;
    download
      .image({
        url: imageUrl,
        dest: `./images/${datePath}.gif`,
      })
      .then(({ filename, image }) => {
        console.log('File saved to', filename);
      })
      .catch(err => {
        console.error(err);
      });

    date.setDate(date.getDate() + 1);
  }
};

const numberToDouble = num => {
  return num < 10 ? '0' + num : num;
};

getImages('2017-09-19', 30);