Millisecond timestamps parsed incorrectly

[latk]

The DateTime::parse_str(...) constructor supports various different formats, but millisecond-granularity timestamps are parsed incorrectly when they have a fractional part.

Testcase to reproduce:

use speedate::DateTime;

fn timestamp_ms(dt: DateTime) -> i64 {
    dt.timestamp() * 1000 + i64::from(dt.time.microsecond / 1000)
}

#[test]
fn from_fractional_float_str() {
    assert_eq!(
        timestamp_ms(DateTime::parse_str("1711445175471.865").unwrap()),
        1711445175471
    )
}

Failure:

---- from_fractional_float_str stdout ----
thread 'from_fractional_float_str' panicked at tests/datetime_timestamp_bytes.rs:9:5:
assertion `left == right` failed
  left: 1711445176335
 right: 1711445175471

Analysis: the difference between the two values is 864, which is close to the fractional part .865. It seems that the fractional part of a float input is always interpreted on the seconds scale, even if the integral part of the input is interpreted on the milliseconds scale.

This is not a bug in the above timestamp_ms() helper. The same problem can be observed when accessing this functionality via Pydantic:

import pydantic  # v2.6.4
from datetime import datetime

class Model(pydantic.BaseModel):
    x: datetime

Model(x="1711445175471.865").x.timestamp() * 1000
#=> 1711445176335.99

Originally I though this was a bug in the custom float parsing which seems to accumulate rounding errors because it skips the checks in f64::from_str(), but I now think the issue is actually in the usage of the DateTime::from_timestamp(ts, micros) constructor by the parse_bytes_with_config() constructor:

speedate/src/datetime.rs

Lines 344 to 346 in ecbe681

	IntFloat::Float(float) => {
	let micro = (float.fract() * 1_000_000_f64).round() as u32;
	Self::from_timestamp_with_config(float.floor() as i64, micro, config)

I am not sure how to fix this. It is impossible to call DateTime::from_timestamp(ts, micros) with a non-zero value for micros without already knowing how the ts part will be interpreted.