date1.diff(date2, 'day') does not produce expected result

[bschwartz757]

Describe the bug
I'm using .diff() with two dayjs objects to calculate a duration. However, this doesn't seem to work with day. Example:

const duration = (start, end) => {
  const daysDiff = end.diff(start, 'day'); // produces `0`, expected `1`
  const hoursDiff = end.diff(start, "hour"); // expected result
  const minutesDiff = end.diff(start, "minute") % 60; // expected result
  return `${daysDiff} day ${hoursDiff} hr ${minutesDiff} mins`;
}

duration("2019-09-28T23:00:00", "2019-09-29T00:00:00")
// -> 1 day 1 hr

However, if I use const daysDiff = end.date() - start.date(); then I get the expected result.
Expected behavior
See above (expect daysDiff to be 1)
-> 1 day 1 hr

Information

• Day.js Version 1.8.16
• OS: Mac OS Mojave Version 10.14.2
• Browser: Chrome Version 77.0.3865.90
• Time zone: GMT-07:00 DST (Pacific Daylight Time)

[iamkun]

const start =  dayjs("2019-09-29T00:00:00")
const end = dayjs("2019-09-28T23:00:00")

end.diff(start, 'day', true); //  -0.0416

If you add true as the third argument, you will get the diff result in float.
In this way, it's easy to find out that the actuall diff in only '0.04' day (1 hour) , so the result 0 is expected. (Math.round(0.04) // 0)

[kthatoto]

@bschwartz757
I had same problem, but I found the solution for yours.

const duration = (start, end) => {
-  const daysDiff = end.diff(start, 'day');
+  const daysDiff = end.startOf('day').diff(start.startOf('day'), 'day'); // produces `1`
  ...
}

I think currently behavior of diff is spec.