Dayjs TZ add 3 months to generate date with day becoming 1st of month

[mattiLeBlanc]

When using a date object created with tz, adding 3 months if not setting the day of the month correctly.
When I revert back to using dayjs() it outputs 4th of October.

dayjs.tz.setDefault('Australia/Sydney');

const today = dayjs().tz();
console.log('TODAY', today.format()); // => 4th of July 2021
console.log('ADD 3 Months', today.add(3, 'month'));  // =>  1th of October

Expected behavior

console.log('TODAY', today.format()); // => 4th of July 2021
console.log('ADD 3 Months', today.add(3, 'month'));  // =>  4th of October

Information

• Day.js Version 1.10.5
• OS: MacOS
• Node 14
• Time zone: [Sydney/Australia]

[mattiLeBlanc]

Strangely, adding 1 or 2 months works fine:

  const today = dayjs();
  console.log('TODAY', today.format());
  console.log('ADD 2 Months', today.add(2, 'month'));
  const today2 = dayjs().tz();
  console.log('TODAY', today2.format());
  console.log('ADD 2 Months', today2.add(2, 'month'));

output:

 console.log
    TODAY 2021-07-04T13:03:24+10:00

      at src/functions/processInvestment/test/process-investment.test.ts:31:11

  console.log
    ADD 2 Months d {
      '$L': 'en',
      '$d': 2021-09-04T03:03:24.704Z,
      '$x': {},
      '$y': 2021,
      '$M': 8,
      '$D': 4,
      '$W': 6,
      '$H': 13,
      '$m': 3,
      '$s': 24,
      '$ms': 704
    }

      at src/functions/processInvestment/test/process-investment.test.ts:32:11

  console.log
    TODAY 2021-07-04T13:03:24+10:00

      at src/functions/processInvestment/test/process-investment.test.ts:34:11

  console.log
    ADD 2 Months d {
      '$L': 'en',
      '$offset': 600,
      '$d': 2021-09-04T03:03:24.731Z,
      '$x': { '$timezone': 'Australia/Sydney' },
      '$y': 2021,
      '$M': 8,
      '$D': 4,
      '$W': 6,
      '$H': 13,
      '$m': 3,
      '$s': 24,
      '$ms': 731
    }

but going to 3+ months it fails:

console.log
    TODAY 2021-07-04T13:03:59+10:00

      at src/functions/processInvestment/test/process-investment.test.ts:31:11

  console.log
    ADD 3 Months d {
      '$L': 'en',
      '$d': 2021-10-04T02:03:59.174Z,
      '$x': {},
      '$y': 2021,
      '$M': 9,
      '$D': 4,
      '$W': 1,
      '$H': 13,
      '$m': 3,
      '$s': 59,
      '$ms': 174
    }

      at src/functions/processInvestment/test/process-investment.test.ts:32:11

  console.log
    TODAY 2021-07-04T13:03:59+10:00

      at src/functions/processInvestment/test/process-investment.test.ts:34:11

  console.log
    ADD 3 Months d {
      '$L': 'en',
      '$offset': 600,
      '$d': 2021-10-01T03:03:59.201Z,
      '$x': { '$timezone': 'Australia/Sydney' },
      '$y': 2021,
      '$M': 9,
      '$D': 1,
      '$W': 5,
      '$H': 13,
      '$m': 3,
      '$s': 59,
      '$ms': 201
    }