[BUG] SPAN has different behavior between pushdown and non-pushdown for timestamp before 1971

[qianheng-aws]

What is the bug?
The PPL:

source = test | eval ts = timestamp("1961-04-12 09:10:00") | stats count() by span(ts, 1h) | head 1

get results:

{
  "schema": [
    {
      "name": "count()",
      "type": "bigint"
    },
    {
      "name": "span(ts,1h)",
      "type": "timestamp"
    }
  ],
  "datarows": [
    [
      6005,
      "1961-04-12 10:00:00"
    ]
  ],
  "total": 1,
  "size": 1
}

"1961-04-12 09:10:00" is rounded to "1961-04-12 10:00:00". However, in DSL, it should be rounded to "1961-04-12 09:00:00"

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

What is the expected behavior?
A clear and concise description of what you expected to happen.

What is your host/environment?

• OS: [e.g. Mac]
• Version [e.g. 3.1.0-SNAPSHOT]
• Plugins SQL

Do you have any screenshots?
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Do you have any additional context?
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