Although I’m not sure whether I could correctly understand your expected result, when I saw the repository of “dayjs”, I noticed that dayjs/src/plugin/isSameOrBefore/index.js. Ref In the case of “isSameOrBefore”, it seems that the script is as follows.

export default (o, c) => {
  c.prototype.isSameOrBefore = function (that, units) {
    return this.isSame(that, units) || this.isBefore(that, units)
  }
}

From your provided document of “IsSameOrBefore”, the sample script is as follows.

var isSameOrBefore = require("dayjs/plugin/isSameOrBefore");
// import isSameOrBefore from 'dayjs/plugin/isSameOrBefore' // ES 2015

dayjs.extend(isSameOrBefore);

dayjs("2010-10-20").isSameOrBefore("2010-10-19", "year");

When these are reflected in Google Apps Script under your showing situation, how about the following modification?

In this answer, it supposes that the library of Daygs you show has already been installed. Please be careful about this.

Modified script:

const { dayjs } = Daygs;

function myFunction() {

  // This is from https://github.com/iamkun/dayjs/blob/dev/src/plugin/isSameOrBefore/index.js
  const isSameOrBefore = (o, c) => {
    c.prototype.isSameOrBefore = function (that, units) {
      return this.isSame(that, units) || this.isBefore(that, units)
    }
  }

  dayjs.extend(isSameOrBefore);
  const res = dayjs("2010-10-20").isSameOrBefore("2010-10-19", "year");
  console.log(res); // true
}

By this, true is showing in the log.

As another plugin, when “IsToday” is used, the sample script is as follows.

const { dayjs } = Daygs;

function myFunction() {

  // This is from https://github.com/iamkun/dayjs/blob/dev/src/plugin/isToday/index.js
  const isToday = (o, c, d) => {
    const proto = c.prototype
    proto.isToday = function () {
      const comparisonTemplate = 'YYYY-MM-DD'
      const now = d()

      return this.format(comparisonTemplate) === now.format(comparisonTemplate)
    }
  }

  dayjs.extend(isToday);
  const res = dayjs().isToday();
  console.log(res); // true
}

Note:

• This modified script is for “IsSameOrBefore”. Unfortunately, I cannot test all plugins. So, I’m not sure whether all plugins can be used with this method. Please be careful about this.

References:

• dayjs at GitHub
• IsSameOrBefore

If you are up for it, feel free to bundle the library from NPM. You can check out my article on this topic.

Here’s the gist:

1. Work locally and make sure you have clasp installed
2. Install and configure Vite as per the article
3. Install the simple Vite plugin from the article
4. Install DayJS from npm with npm i dayjs
5. Import DayJS in your code with ES6 syntax as you would in normal JavaScript
6. Build your code with Vite and push it with clasp