dayjs.tz().diff() different results on different local timezones

[thekip]

Describe the bug

The test case

      import dayjs from 'dayjs';
      import utc from 'dayjs/plugin/utc';
      import timezone from 'dayjs/plugin/timezone';

      dayjs.extend(utc);
      dayjs.extend(timezone);

      dayjs.tz.setDefault('America/New_York');

      const startDate = dayjs.tz('2020-10-01');
      const endDate = dayjs.tz('2020-10-30');

      console.log({
        TZ: process.env.TZ,
        diff: endDate.diff(startDate, 'day'),
        start_date_c_month: startDate.toISOString(),
        end_date_c_month: endDate.toISOString(),
      });

Than run node.js with TZ environment var:

env TZ=UTC node test.js

    {
      TZ: 'UTC',
      diff: 29,
      start_date_c_month: '2020-10-01T04:00:00.000Z',
      end_date_c_month: '2020-10-30T04:00:00.000Z'
    }

#########

env TZ=CET node test.js

 console.log
    {
      TZ: 'CET',
      diff: 28,
      start_date_c_month: '2020-10-01T04:00:00.000Z',
      end_date_c_month: '2020-10-30T04:00:00.000Z'
    }

With different local timezone, Dayjs.diff produce different results even if timezone explicitly set.
Probably it just doesn't take timezone into account while calculation day boundaries.

I found this during development node.js application, but think that this could be reproduced in browser as well (but it's not easy to change a browser timezone on the fly)

possible related #1212

Expected behaviour
The results should be consistent and not depend on local timezone.

Information

• Day.js Version 1.9.4
• OS: macOs 10.15.7
• Browser Node 12
• Time zone: diffrent, see node invocations

[felipeclopes]

+1

[RamiTae]

+1