Duration between dates seems to be incorrect

Describe the bug Duration seems to be incorrect. The difference between March 29th, 2020 and March 28th 2021 should be less than 12 months

Expected behavior Calculating the difference between two days one year apart from each other should yield a difference of one year.

Information

• Day.js Version: 1.10.3
• OS: Windows 10 20H2
• Browser Chrome 89
• Time zone: GMT -4:00

I’m trying to get the difference between two dates:

• A fixed date (March 29th, 2020)
• Todays date (at the time of writing March 28th, 2021)

I do that with:

      const lockdown = dayjs("3/29/2020", "M/DD/YYYYY"); //29th March 2020
      const currentDate = dayjs();
 
      // get the difference between the moments
      const diff = currentDate.diff(lockdown);
    
      //express as a duration
      const diffDuration = dayjs.duration(diff);

   
      // display
      const timeInLockDown = {
        months: diffDuration.months(),
        days: diffDuration.days(),
        hours: diffDuration.hours(),
        minutes: diffDuration.minutes(),
        seconds: diffDuration.seconds()
      };

This yields diffDuration as:

days: 4
hours: 1
milliseconds: 709
minutes: 4
months: 12
seconds: 5
years: 0

Which doesn’t make sense to me? It shouldn’t 12 months and 4 days. Also, I think that if it is 12 months and 4 days, then years should be 1 but in this instance it’s 0.

Is this expected behaviour?

[avand]

I just noticed a bug and it seems related to this issue:

// dayjs version 1.10.5
import dayjs from 'dayjs';
import dayjsDurationPlugin from 'dayjs/plugin/duration';

const a = dayjs('3/19/2018');
const b = dayjs('3/17/2021');
const diff = b.diff(a, 'day'); // => 1094 correct
dayjs.duration(diff, 'days'); // => { years: 2, months: 12, days: 4 } incorrect

I would expect the output to be { years: 3, days: 4 }.

[guypursey]

Further evidence for the above observed in comment on a related issue.

Brief thread on more accurate duration requirements also adds to this.