How to reproduce

• ClickHouse server: 21.4.7.3
• Queries

  WITH toDateTime64('1940-01-01 01:00:00', 0) AS dt64
  SELECT dt64,toStartOfDay(dt64)
  

• Results

  ┌────────────────dt64─┬──toStartOfDay(dt64)─┐
  │ 1940-01-01 01:00:00 │ 2076-02-06 06:28:16 │
  └─────────────────────┴─────────────────────┘
  

Expected behavior
return date 1940-01-01 00:00:00 instead of 2076-02-06 06:28:16

Additional context
most date functions return unexpected value
this seems to overflowed by date before 1970