Strange behaviour on pre 1911 dates and Portuguese calendar

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**Describe the bug**
I am on a computer configured to use Portuguese calendar.

If I parse '1910-10-25' date, I get the expected ISO formatted date:
dayjs('1910-10-25').format()
==> '1910-10-25T00:00:00-00:30'

If I parse it again, I am getting a different date (notice the minutes)
dayjs('1910-10-25T00:00:00-00:30').format()
==> '1910-10-24T23:53:15-00:30'

If I keep doing this, the date loses 7 minutes on each iteration:
dayjs('1910-10-24T23:53:15-00:30').format()
==> '1910-10-24T23:46:30-00:30'

**Expected behavior**
I expected the output to be the same as the input:
dayjs('1910-10-25T00:00:00-00:30').format()
==> '1910-10-25T00:00:00-00:30'

**Information**
- OS: Windows
- Browser: Chrome
- Time zone: GMT

As far as I can tell, this might be caused by the offset used in portugal prior to may 1911: GMT–00:36:44
difference to -00:30 is around 7 minutes.

Am I doing something wrong here or is this a bug and can be fixed?

Start by reproducing the issue with the shown dayjs parsing and format calls for the Portuguese calendar and pre-1911 date. Trace the date parsing and offset handling used by these calls, then add regression coverage ensuring repeated formatting preserves the input timestamp and offset.

Written by the indexing model from the issue text.