Converting a 12h clock to 24h clock

use std::io::{stdin, Error, ErrorKind, Result};

struct Time {
    hour: u8,
    minute: u8,
    second: u8,
}

impl Time {
    pub fn from_12h(time: &str) -> Result<Time> {
        assert!(time.chars().count() == 10);
        let mut chars = time.chars();

        let hour: String = chars.by_ref().take(2).collect();
        let minute: String = chars.by_ref().skip(1).take(2).collect();
        let second: String = chars.by_ref().skip(1).take(2).collect();
        let period: String = chars.by_ref().take(2).collect();

        Time::hour_from_12h_to_24h(hour.parse().unwrap(), &period).map(|hour_24h| {
            Time {
                hour: hour_24h,
                minute: minute.parse().unwrap(),
                second: second.parse().unwrap(),
            }
        })
    }

    fn hour_from_12h_to_24h(hour: u8, period: &str) -> Result<u8> {
        match period {
            "AM" => Ok(hour % 12),
            "PM" => Ok(hour % 12 + 12),
            _ => Err(Error::new(ErrorKind::InvalidData, format!("invalid period {}", period))),
        }
    }

    pub fn to_24h(&self) -> String {
        format!("{:02}:{:02}:{:02}", self.hour, self.minute, self.second)
    }
}

fn main() {
    let time_12h = read_12h_time().expect("input error");
    let time = Time::from_12h(&time_12h).expect("error");

    println!("{}", time.to_24h());
}

fn read_12h_time() -> Result<String> {
    let mut buffer = String::new();
    match read_line(&mut buffer) {
        Ok(10) => Ok(buffer),
        Ok(length) => {
            Err(Error::new(ErrorKind::InvalidInput,
                           format!("expected 10 characters, got {}", length)))
        }
        Err(e) => Err(e),
    }
}

fn read_line(buffer: &mut String) -> Result<usize> {
    stdin().read_line(buffer)
}

#[macro_use]
extern crate quick_error;

use std::str::FromStr;
use std::io::stdin;
use std::fmt;

quick_error! {
    #[derive(Debug, Clone, PartialEq)]
    enum Error {
        MissingMeridian {
            description("missing the meridian")
            display("Missing the meridian")
        }
        InvalidMeridian(what: String) {
            description("invalid meridian")
            display("Invalid meridian: {}", what)
        }
        MissingComponent(which: TimeComponent) {
            description("missing a time component")
            display("Missing {}", which)
        }
        InvalidComponent(which: TimeComponent, why: std::num::ParseIntError) {
            description("invalid time component")
            display("Invalid {}: {}", which, why)
            cause(why)
        }
    }
}

type Result<T> = std::result::Result<T, Error>;

#[derive(Debug, Copy, Clone, PartialEq)]
enum Meridian {
    AnteMeridian,
    PostMeridian,
}

#[derive(Debug, Copy, Clone, PartialEq)]
enum TimeComponent {
    Hour,
    Minutes,
    Seconds,
}

impl fmt::Display for TimeComponent {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use TimeComponent::*;

        let name = match *self {
            Hour => "hour",
            Minutes => "minutes",
            Seconds => "seconds",
        };
        write!(f, "{}", name)
    }
}

impl FromStr for Meridian {
    type Err = Error;

    fn from_str(s: &str) -> Result<Meridian> {
        match s {
            "AM" => Ok(Meridian::AnteMeridian),
            "PM" => Ok(Meridian::PostMeridian),
            _ => Err(Error::InvalidMeridian(s.to_string())),
        }
    }
}

#[derive(Debug, Copy, Clone, PartialEq)]
struct TwelveHourTime {
    hour: u8,
    minutes: u8,
    seconds: u8,
    meridian: Meridian,
}

impl FromStr for TwelveHourTime {
    type Err = Error;

    fn from_str(time: &str) -> Result<TwelveHourTime> {
        use Error::*;
        use TimeComponent::*;

        let mut parts = time.splitn(3, ":");

        let hour = parts.next().ok_or(MissingComponent(Hour))?;
        let minutes = parts.next().ok_or(MissingComponent(Minutes))?;
        let seconds_meridian = parts.next().ok_or(MissingComponent(Seconds))?;

        if seconds_meridian.len() < 4 {
            return Err(MissingMeridian);
        }

        let (seconds, meridian) = seconds_meridian.split_at(2);

        let hour = hour.parse().map_err(|e| InvalidComponent(Hour, e))?;
        let minutes = minutes.parse().map_err(|e| InvalidComponent(Minutes, e))?;
        let seconds = seconds.parse().map_err(|e| InvalidComponent(Seconds, e))?;
        let meridian = meridian.parse()?;

        Ok(TwelveHourTime {
            hour: hour,
            minutes: minutes,
            seconds: seconds,
            meridian: meridian,
        })
    }
}

#[derive(Debug, Copy, Clone, PartialEq)]
struct TwentyFourHourTime {
    hour: u8,
    minutes: u8,
    seconds: u8,
}

impl From<TwelveHourTime> for TwentyFourHourTime {