Strange behaviour with dates between 1969 to 1971

**Describe the bug**
When using dates in the years 1969, 1970 and 1971, using operations like "startOf" results in times from other days
i.e. `dayjs('1970-01-07T01:00:00').startOf('day')` will result in "Tue, 06 Jan 1970 23:00:00 GMT". Years before and after these 3 years do not show the same problem

**Expected behavior**
`dayjs('1970-01-07T01:00:00').startOf('day')` should display "Weds, 07 Jan 1970 00:00:00 GMT"

**Information**
- Day.js Version: 1.11.0
- OS: iOS
- Browser: chrome 131.0.6778.265
- Time zone: GMT

Reproduce the reported dayjs('1970-01-07T01:00:00').startOf('day') call in the GMT environment, then compare its result with dates before and after 1969–1971. Start at the startOf entry point and verify that these years produce the expected calendar day without regressing other years.

Written by the indexing model from the issue text.