Extra slow Utc to Local conversion

[i-sinister]

I have to read large parquet files with 500M-1B records where one of the columns is a datetime. By convention for parquet timestamps are saved as Int96 UTC timestamp. When reading file I have to convert it to local time to make some simple calculations. Reading Int96 from file, converting it to milliseconds, creating DateTime<Utc> operations are fast, however converting to Local timezone is incredibly slow:

use std::time::Instant;
use chrono::{DateTime, Local, Utc};

fn create_timestamps() -> Vec<i64> {
    let mut timestamps = Vec::with_capacity(5_000_000);
    for _ in 0..100_000 {
        for j in 0..50 {
            timestamps.push(1_716_757_200_000 + 60_000 * j);
	}
    }
    timestamps
}

fn convert_to_utc(timestamps: &Vec<i64>, dates: &mut Vec<DateTime<Utc>>) {
    for &timestamp in timestamps {
	dates.push(DateTime::<Utc>::from_timestamp_millis(timestamp).unwrap());
    }
}

fn convert_to_local(timestamps: &Vec<i64>, dates: &mut Vec<DateTime<Local>>) {
    for &timestamp in timestamps {
        dates.push(DateTime::<Utc>::from_timestamp_millis(timestamp).unwrap().with_timezone(&Local));
    }
}

fn main() {
    //  chrono = "0.4.38"
    let timestamps = create_timestamps();
    let mut utc_dates = Vec::with_capacity(5_000_000);
    let start_time = Instant::now();
    // emulate reading 500M rows
    for _ in 0..100 {
        convert_to_utc(&timestamps, &mut utc_dates);
        utc_dates.clear();
    }
    let elapsed_time = start_time.elapsed();
    println!("convert to utc: {:?}", elapsed_time);
    let mut local_dates = Vec::with_capacity(5_000_000);
    let start_time = Instant::now();
    // emulate reading 500M rows
    for _ in 0..100 {
        convert_to_local(&timestamps, &mut local_dates);
        local_dates.clear();
    }
    let elapsed_time = start_time.elapsed();
    println!("convert to local: {:?}", elapsed_time);
}

> cargo --version
cargo 1.78.0 (54d8815d0 2024-03-26)
> rustc --version
rustc 1.78.0 (9b00956e5 2024-04-29)
>  cargo run --release
    Finished `release` profile [optimized] target(s) in 0.01s
     Running `target\release\chrono-tz-convert.exe`
convert to utc: 5.517348s
convert to local: **119.8019992s**

Here is c# version for comparison (note that it is x6 times faster):

namespace DateTimeBenchMark
{
	internal class Program
	{
		static void Main(string[] args)
		{

			var timestamps = new List<long>(5_000_000);
			for (var i = 0; i < 100_000; i++)
			{
				for (var j = 0; j < 50; j++)
				{
					timestamps.Add(1_716_757_200_000 + 60_000 * j);
				}
			}

			var utc_datetimes = new List<DateTime>(5_000_000);
			var stopwatch = System.Diagnostics.Stopwatch.StartNew();
			for (var i = 0; i < 100; i++)
			{
				foreach (var timestamp in timestamps)
				{
					utc_datetimes.Add(DateTime.UnixEpoch.AddMicroseconds(timestamp));
				}
			}

			stopwatch.Stop();
			Console.WriteLine($"convert to utc: {stopwatch.Elapsed}");
			var local_datetimes = new List<DateTime>(5_000_000);
			stopwatch.Restart();
			for (var i = 0; i < 100; i++)
			{
				foreach (var timestamp in timestamps)
				{
					local_datetimes.Add(DateTime.UnixEpoch.AddMicroseconds(timestamp).ToLocalTime());
				}

				local_datetimes.Clear();
			}

			stopwatch.Stop();
			Console.WriteLine($"convert to local: {stopwatch.Elapsed}");
		}
	}
}

convert to utc: 00:00:03.2969532
convert to local: **00:00:19.5144110**