Calculate days, months, years between two dates

dateDiff.js

function dateDiff_COMMENTED(date1, date2) {
      let days = 0;
      let months = 0;
      let years = 0;
      // The calculations need date1 to be earlier than date2.
      // If date2 is earlier, swap date1 and date2
      if (date1 > date2) [date2, date1] = [date1, date2];
      
      let year1 = date1.getFullYear();
      let year2 = date2.getFullYear();
      let month1 = date1.getMonth();
      let month2 = date2.getMonth();
      let day1 = date1.getDate();
      let day2 = date2.getDate();

      /* Minus date1 and date2s year, month and day. 
        Best case, date2s year, month and day are all >= date1s year, month and day, respectively, 
        and our answer is the result of these 3 sums.
        date1 = 2021, 4, 4
        date2 = 2021, 8, 5
        years = 2021 - 2021
        months = 8 - 4
        days = 5 - 4
        Answer: { days: 1, months: 4, years: 0 }
      */
      years = year2 - year1;
      months = month2 - month1;
      days = day2 - day1;

      if (days < 0) {
        /*
          date1 = 2021, 4, 4
          date2 = 2021, 8, 1
          We would count months like
          1 month ahead = 2021, 5, 4
          2 months ahead = 2021, 6, 4
          3 months ahead = 2021, 7, 4
          4 months ahead = 2021, 8, 4
          But 4 months ahead is past date2, so we say 3 months ahead and then calculate the days from 
          the 4th day to the end of the 7th month, plus the days in date2.
        */
        months -= 1;
        /*           
          new Date(date2.getFullYear(), date2.getMonth() - 1, 0).getDate() will return the number of days
          in the month previous to date2s month.

          In our example, there are 31 days in the 7th month, date1.days is 4 and date2.days is 1
          days = 31 - 4 + 1
          Answer = { days: 28, months: 3, years: 0 }
        */
        days = new Date(date2.getFullYear(), date2.getMonth() - 1, 0).getDate() - day1 + day2;
      }
      if (months < 0) {
        /* If months are negative we roll back a year
          date1 = 2021, 4, 4
          date2 = 2022, 3, 5
          months = 3 - 4
          So we know it's not a full year, lets minus 1 year and add on 12 months
          months += 12
          So -1 + 12 = 11
          { months: 11 }
        */
        months += 12;
        years -= 1;
      }
      return { days, months, years };
    }

    function dateDiff(date1, date2) {
      let days = 0;
      let months = 0;
      let years = 0;

      if (date1 > date2) [date2, date1] = [date1, date2];
      
      let year1 = date1.getFullYear();
      let year2 = date2.getFullYear();
      let month1 = date1.getMonth();
      let month2 = date2.getMonth();
      let day1 = date1.getDate();
      let day2 = date2.getDate();

      years = year2 - year1;
      months = month2 - month1;
      days = day2 - day1;

      if (days < 0) {
        months -= 1;
        days = new Date(date2.getFullYear(), date2.getMonth() - 1, 0).getDate() - day1 + day2;
      }
      if (months < 0) {
        months += 12;
        years -= 1;
      }
      return { days, months, years };
    }



    /*
      Everything below is just for testing the above code
    */
    function c(a, v) {
      if (a.days === v.days && a.months === v.months && a.years === v.years) return true;
      return `${JSON.stringify(a)} !== ${JSON.stringify(v)}`;
    }

    [ [new Date(2022, 3, 3), { days: 27, months: 10, years: 0}],
      [new Date(2022, 3, 5), { days: 1, months: 11, years: 0}],
      [new Date(2021, 5, 1), { days: 27, months: 0, years: 0}],
      [new Date(2021, 5, 5), { days: 1, months: 1, years: 0}],
      [new Date(2021, 6, 1), { days: 28, months: 1, years: 0}],
      [new Date(2021, 6, 5), { days: 1, months: 2, years: 0}],
      [new Date(2021, 7, 1), { days: 27, months: 2, years: 0}],
      [new Date(2021, 7, 5), { days: 1, months: 3, years: 0}],
      [new Date(2021, 8, 1), { days: 28, months: 3, years: 0}],
      [new Date(2021, 8, 5), { days: 1, months: 4, years: 0}],
      [new Date(2021, 9, 1), { days: 28, months: 4, years: 0}],
      [new Date(2021, 9, 5), { days: 1, months: 5, years: 0}],
      [new Date(2021, 10, 1), { days: 27, months: 5, years: 0}],
      [new Date(2021, 10, 5), { days: 1, months: 6, years: 0}],
      [new Date(2021, 11, 1), { days: 28, months: 6, years: 0}],
      [new Date(2021, 11, 5), { days: 1, months: 7, years: 0}],
      [new Date(2021, 12, 1), { days: 27, months: 7, years: 0}],
      [new Date(2021, 12, 5), { days: 1, months: 8, years: 0}],
      [new Date(2022, 1, 1), { days: 28, months: 8, years: 0}],
      [new Date(2022, 1, 5), { days: 1, months: 9, years: 0}],
      [new Date(2022, 2, 1), { days: 28, months: 9, years: 0}],
      [new Date(2022, 2, 5), { days: 1, months: 10, years: 0}],
      [new Date(2022, 3, 1), { days: 25, months: 10, years: 0}],
      [new Date(2022, 3, 5), { days: 1, months: 11, years: 0}],
      [new Date(2022, 4, 1), { days: 28, months: 11, years: 0}],
      [new Date(2022, 4, 5), { days: 1, months: 0, years: 1}],
      [new Date(2022, 5, 1), { days: 27, months: 0, years: 1}],
      [new Date(2022, 5, 5), { days: 1, months: 1, years: 1}],
      [new Date(2021, 4, 4, 10), { days: 0, months: 0, years: 0}],
      [new Date(2020, 8, 1), { days: 3, months: 8, years: 0}]
    ].forEach(test => {
      console.log(c(dateDiff(new Date(2021, 4, 4), test[0]), test[1]));
    });