dayjs(timestamp).startOf('day') wrong result when timestamp < 1000000000

**Describe the bug**
when timestamp < 1000000000,
startOf('day') output wrong

**Expected behavior**
dayjs(999999999)
result : g3 {$L: 'en', $u: undefined, $d: Sun Sep 09 2001 09:46:39 GMT+0800 (中国标准时间), $x: {…}, $y: 2001, …}

dayjs(999999999).startOf('day')
result g3 {$L: 'en', $u: undefined, $d: Thu Aug 16 33657 00:00:00 GMT+0800 (中国标准时间), $x: {…}, $y: 33657, …}

Start by tracing the dayjs(timestamp).startOf('day') entry point with the reported value 999999999. Compare the resulting date with the expected September 9, 2001 day boundary, and consider the issue complete when startOf('day') returns the correct start date for timestamps below 1000000000.

Written by the indexing model from the issue text.