date-competence.js

const moment = require("moment");

const START = 25;
const DAY = parseInt(moment().format("DD"));

let start, end;

if (DAY >= START) {
  start = moment().format("YYYY-MM-25");
  end = moment().add(1, "M").format("YYYY-MM-24");
} else {
  start = moment().subtract(1, "M").format("YYYY-MM-25");
  end = moment().format("YYYY-MM-24");
}

console.log({ start, end });
// today: 01/10/2020
// { start: '2020-09-25', end: '2020-10-24' }