diff --git a/src/core/error.rs b/src/core/error.rs
index c11221e..c5289a6 100644
--- a/src/core/error.rs
+++ b/src/core/error.rs
@@ -6,9 +6,11 @@ use actix_web::{
 use serde::{ser::SerializeMap, Serialize, Serializer};
 use std::{fmt, io};
 
-use crate::util::{crypto, validate};
+use crate::util::{crypto, validate, xsd};
 
 pub type Result<T> = std::result::Result<T, Error>;
+pub use std::error::Error as StdError;
+pub use std::result::Result as StdResult;
 
 #[derive(Debug)]
 pub enum Error {
@@ -27,6 +29,7 @@ pub enum Error {
     MalformedHeader(header::ToStrError),
     NotFound,
     Reqwest(reqwest::Error),
+    Xsd(xsd::FromStrError),
 }
 
 impl ResponseError for Error {
@@ -43,6 +46,7 @@ impl ResponseError for Error {
             Error::MalformedApub(_) => StatusCode::UNPROCESSABLE_ENTITY,
             Error::MalformedHeader(_) => StatusCode::BAD_REQUEST,
             Error::NotFound => StatusCode::NOT_FOUND,
+            Error::Xsd(_) => StatusCode::UNPROCESSABLE_ENTITY,
             _ => StatusCode::INTERNAL_SERVER_ERROR,
         }
     }
@@ -52,8 +56,8 @@ impl ResponseError for Error {
     }
 }
 
-impl std::error::Error for Error {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+impl StdError for Error {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
         match self {
             Error::BadToken(e) => Some(e),
             Error::Crypto(e) => Some(e),
@@ -85,6 +89,7 @@ impl fmt::Display for Error {
             Error::MalformedApub(msg) => write!(f, "Malformed ActivityPub: {msg}"),
             Error::MalformedHeader(to_str_error) => to_str_error.fmt(f),
             Error::Reqwest(reqwest_error) => reqwest_error.fmt(f),
+            Error::Xsd(xsd_error) => xsd_error.fmt(f),
         }
     }
 }
@@ -134,6 +139,12 @@ impl From<crypto::Error> for Error {
     }
 }
 
+impl From<xsd::FromStrError> for Error {
+    fn from(e: xsd::FromStrError) -> Error {
+        Error::Xsd(e)
+    }
+}
+
 impl Serialize for Error {
     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
     where
diff --git a/src/util/mod.rs b/src/util/mod.rs
index 59914b1..c188ee5 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -14,3 +14,5 @@ pub mod token;
 pub mod transcode;
 /// Validation framework.
 pub mod validate;
+/// Some XSD types and parsers.
+pub mod xsd;
diff --git a/src/util/xsd/duration.rs b/src/util/xsd/duration.rs
new file mode 100644
index 0000000..e96d95b
--- /dev/null
+++ b/src/util/xsd/duration.rs
@@ -0,0 +1,492 @@
+use std::fmt;
+use std::fmt::Write;
+use std::str::FromStr;
+
+use crate::core::*;
+use crate::util::slice::SliceCursor;
+use crate::util::xsd::{from_str_error, FromStrError};
+
+#[derive(Clone, Eq, PartialEq, Default, Debug)]
+pub struct Duration {
+    negative: bool,
+    years: u16,
+    months: u32,
+    days: u32,
+    hours: u32,
+    mins: u32,
+    millis: u64,
+}
+
+enum DateUnit {
+    Years(u16),
+    Months(u16),
+    Days(u32),
+}
+
+enum TimeUnit {
+    Hours(u32),
+    Minutes(u32),
+    Millis(u64),
+}
+
+trait UnitDomain: Sized {
+    fn index(&self) -> usize;
+    fn val(&self) -> u64;
+    fn parse(cursor: &mut SliceCursor<u8>) -> StdResult<Self, FromStrError>;
+}
+
+impl Duration {
+    pub fn is_negative(&self) -> bool {
+        self.negative
+    }
+
+    pub fn set_negative(&mut self, negative: bool) {
+        self.negative = negative;
+    }
+
+    pub fn years(&self) -> u16 {
+        self.years
+    }
+
+    pub fn set_years(&mut self, years: u16) {
+        self.years = years;
+    }
+
+    pub fn from_years(years: u16) -> Self {
+        Self {
+            years,
+            ..Default::default()
+        }
+    }
+
+    pub fn months(&self) -> u32 {
+        self.months
+    }
+
+    pub fn set_months(&mut self, months: u32) {
+        self.months = months;
+    }
+
+    pub fn from_months(months: u32) -> Self {
+        Self {
+            months,
+            ..Default::default()
+        }
+    }
+
+    pub fn days(&self) -> u32 {
+        self.days
+    }
+
+    pub fn set_days(&mut self, days: u32) {
+        self.days = days;
+    }
+
+    pub fn from_days(days: u32) -> Self {
+        Self {
+            days,
+            ..Default::default()
+        }
+    }
+
+    pub fn hours(&self) -> u32 {
+        self.hours
+    }
+
+    pub fn set_hours(&mut self, hours: u32) {
+        self.hours = hours;
+    }
+
+    pub fn from_hours(hours: u32) -> Self {
+        Self {
+            hours,
+            ..Default::default()
+        }
+    }
+
+    pub fn mins(&self) -> u32 {
+        self.mins
+    }
+
+    pub fn set_mins(&mut self, mins: u32) {
+        self.mins = mins;
+    }
+
+    pub fn from_mins(mins: u32) -> Self {
+        Self {
+            mins,
+            ..Default::default()
+        }
+    }
+
+    pub fn secs(&self) -> u64 {
+        self.millis / 1000
+    }
+
+    pub fn set_secs(&mut self, secs: u64) {
+        self.millis = secs * 1000 + self.millis % 1000;
+    }
+
+    pub fn from_secs(secs: u64) -> Self {
+        Self {
+            millis: secs * 1000,
+            ..Default::default()
+        }
+    }
+
+    pub fn millis(&self) -> u16 {
+        (self.millis % 1000) as u16
+    }
+
+    pub fn set_millis(&mut self, millis: u16) {
+        debug_assert!(millis < 1000);
+        self.millis = self.secs() * 1000 + millis as u64;
+    }
+
+    pub fn from_millis(millis: u64) -> Self {
+        Self {
+            millis,
+            ..Default::default()
+        }
+    }
+}
+
+impl FromStr for Duration {
+    type Err = FromStrError;
+
+    fn from_str(s: &str) -> StdResult<Self, Self::Err> {
+        let mut cursor = SliceCursor::new(s.as_bytes());
+
+        let negative = cursor.next_if(|&b| b == b'-').is_some();
+        cursor
+            .next_if(|&b| b == b'P')
+            .ok_or_else(|| from_str_error("Expected a 'P'"))?;
+        let mut vals: [Option<u64>; 6] = [None; 6];
+        let mut prev_index = None;
+
+        parse_unit_domain::<DateUnit>(&mut vals, &mut prev_index, &mut cursor)?;
+        match cursor.peek().copied() {
+            Some(b'T') => {
+                cursor.next();
+                let prev_index_before = prev_index;
+                parse_unit_domain::<TimeUnit>(&mut vals, &mut prev_index, &mut cursor)?;
+                if prev_index_before == prev_index {
+                    return Err(from_str_error("Time separator must be followed by a value"));
+                }
+            }
+            Some(_) => return Err(from_str_error("Syntax error")),
+            None => {}
+        };
+        if prev_index.is_none() {
+            return Err(from_str_error("No values specified"));
+        }
+        if cursor.next().is_some() {
+            return Err(from_str_error("Unexpected trailing character"));
+        }
+
+        Ok(Self {
+            negative,
+            years: convert_to(vals[0], "Year value exceeds range")?,
+            months: convert_to(vals[1], "Months exceed range")?,
+            days: convert_to(vals[2], "Days exceed range")?,
+            hours: convert_to(vals[3], "Hours exceed range")?,
+            mins: convert_to(vals[4], "Minutes exceed range")?,
+            millis: vals[5].unwrap_or_default(),
+        })
+    }
+}
+
+fn parse_unit_domain<T: UnitDomain>(
+    vals: &mut [Option<u64>; 6],
+    prev_index: &mut Option<usize>,
+    cursor: &mut SliceCursor<u8>,
+) -> StdResult<(), FromStrError> {
+    for _ in 0..3 {
+        match cursor.peek().copied() {
+            Some(b'T') => break,
+            None => break,
+            Some(_) => {
+                let val = T::parse(cursor)?;
+                let index = val.index();
+                if Some(index) < *prev_index {
+                    return Err(from_str_error("Wrong unit order"));
+                }
+                *prev_index = Some(index);
+
+                if vals[val.index()].replace(val.val()).is_some() {
+                    return Err(from_str_error("Duplicate unit definition"));
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn convert_to<T, U>(t: Option<T>, msg: impl Into<String>) -> StdResult<U, FromStrError>
+where
+    T: TryInto<U> + Default,
+{
+    t.unwrap_or_default()
+        .try_into()
+        .map_err(|_| from_str_error(msg))
+}
+
+impl fmt::Display for Duration {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.negative {
+            f.write_char('-')?;
+        }
+        f.write_char('P')?;
+        if self.years == 0
+            && self.months == 0
+            && self.days == 0
+            && self.hours == 0
+            && self.mins == 0
+            && self.millis == 0
+        {
+            return f.write_str("T0S");
+        }
+
+        if self.years > 0 {
+            f.write_fmt(format_args!("{}Y", self.years))?;
+        }
+        if self.months > 0 {
+            f.write_fmt(format_args!("{}M", self.months))?;
+        }
+        if self.days > 0 {
+            f.write_fmt(format_args!("{}D", self.days))?;
+        }
+        if self.hours as u64 + self.mins as u64 + self.millis > 0 {
+            f.write_char('T')?;
+            if self.hours > 0 {
+                f.write_fmt(format_args!("{}H", self.hours))?;
+            }
+            if self.mins > 0 {
+                f.write_fmt(format_args!("{}M", self.mins))?;
+            }
+            if self.millis > 0 {
+                f.write_fmt(format_args!("{}", self.millis / 1000))?;
+                if self.millis % 1000 > 0 {
+                    f.write_fmt(format_args!(".{:03}", self.millis % 1000))?;
+                }
+                f.write_fmt(format_args!("S"))?;
+            }
+        }
+        Ok(())
+    }
+}
+
+impl UnitDomain for DateUnit {
+    fn index(&self) -> usize {
+        match self {
+            DateUnit::Years(_) => 0,
+            DateUnit::Months(_) => 1,
+            DateUnit::Days(_) => 2,
+        }
+    }
+
+    fn val(&self) -> u64 {
+        match self {
+            DateUnit::Years(n) => *n as u64,
+            DateUnit::Months(n) => *n as u64,
+            DateUnit::Days(n) => *n as u64,
+        }
+    }
+
+    fn parse(cursor: &mut SliceCursor<u8>) -> StdResult<Self, FromStrError> {
+        let n: u32 = parse_int(cursor)?;
+        match cursor.next().copied() {
+            Some(b'Y') => {
+                Ok(DateUnit::Years(n.try_into().map_err(|e| {
+                    from_str_error(format!("Invalid years value: {e}"))
+                })?))
+            }
+            Some(b'M') => {
+                Ok(DateUnit::Months(n.try_into().map_err(|e| {
+                    from_str_error(format!("Invalid months value: {e}"))
+                })?))
+            }
+            Some(b'D') => Ok(DateUnit::Days(n)),
+            Some(_) => Err(from_str_error("Invalid unit specifier")),
+            None => Err(from_str_error("Missing unit specifier")),
+        }
+    }
+}
+
+impl UnitDomain for TimeUnit {
+    fn index(&self) -> usize {
+        match self {
+            TimeUnit::Hours(_) => 3,
+            TimeUnit::Minutes(_) => 4,
+            TimeUnit::Millis(_) => 5,
+        }
+    }
+
+    fn val(&self) -> u64 {
+        match self {
+            TimeUnit::Hours(n) => *n as u64,
+            TimeUnit::Minutes(n) => *n as u64,
+            TimeUnit::Millis(n) => *n,
+        }
+    }
+
+    fn parse(cursor: &mut SliceCursor<u8>) -> StdResult<Self, FromStrError> {
+        let int: u64 = parse_int(cursor)?;
+        let decimals = cursor
+            .next_if(|&c| c == b'.')
+            .and_then(|_| {
+                let fract_bytes = cursor.next_while(|c| c.is_ascii_digit());
+                // only support up to 3 decimal digits
+                let fract_bytes = &fract_bytes[..fract_bytes.len().min(4)];
+                std::str::from_utf8(fract_bytes)
+                    .ok()?
+                    .parse::<u32>() // this fails if the string is empty
+                    .ok()
+                    .map(|f| f * 10u32.pow(3 - fract_bytes.len() as u32))
+            })
+            .map(|n| n as u64);
+
+        match cursor.next().copied() {
+            Some(b'H') => {
+                if decimals.is_none() {
+                    Ok(Self::Hours(int.try_into().map_err(|e| {
+                        from_str_error(format!("Invalid hours value: {e}"))
+                    })?))
+                } else {
+                    Err(from_str_error("Hours cannot contain a fractional part"))
+                }
+            }
+            Some(b'M') => {
+                if decimals.is_none() {
+                    Ok(Self::Minutes(int.try_into().map_err(|e| {
+                        from_str_error(format!("Invalid minutes value: {e}"))
+                    })?))
+                } else {
+                    Err(from_str_error("Minutes cannot contain a fractional part"))
+                }
+            }
+            Some(b'S') => int
+                .checked_mul(1000)
+                .and_then(|ms| ms.checked_add(decimals.unwrap_or(0)))
+                .map(Self::Millis)
+                .ok_or_else(|| from_str_error("Integer overflow while parsing seconds")),
+            Some(_) => Err(from_str_error("Invalid unit specified")),
+            None => Err(from_str_error("Missing unit specifier")),
+        }
+    }
+}
+
+fn parse_int<F>(cursor: &mut SliceCursor<u8>) -> StdResult<F, FromStrError>
+where
+    F: FromStr,
+    F::Err: fmt::Display,
+{
+    let num = std::str::from_utf8(cursor.next_while(|&c| c.is_ascii_digit()))
+        .expect("ASCII digits are always valid UTF-8");
+    num.parse::<F>()
+        .map_err(|e| from_str_error(format!("Cannot parse number: {e}")))
+}
+
+impl fmt::Display for FromStrError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.msg)
+    }
+}
+
+impl StdError for FromStrError {
+    fn description(&self) -> &str {
+        &self.msg
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Duration;
+    use std::str::FromStr;
+
+    #[test]
+    fn set_vals() {
+        let mut duration = Duration::default();
+
+        duration.set_years(1);
+        assert_eq!(duration.years(), 1);
+        duration.set_months(2);
+        assert_eq!(duration.months(), 2);
+        duration.set_days(3);
+        assert_eq!(duration.days(), 3);
+        duration.set_hours(4);
+        assert_eq!(duration.hours(), 4);
+        duration.set_mins(5);
+        assert_eq!(duration.mins(), 5);
+        duration.set_secs(6);
+        assert_eq!(duration.secs(), 6);
+        duration.set_millis(7);
+        assert_eq!(duration.millis(), 7);
+        assert_eq!(duration.secs(), 6);
+    }
+
+    #[test]
+    fn from_str() {
+        let test_values = [
+            (
+                "-P1Y2M3DT4H5M6.7S",
+                (true, 1u16, 2u32, 3u32, 4u32, 5u32, 6u64, 700u16),
+                "-P1Y2M3DT4H5M6.700S",
+            ),
+            (
+                "P7Y6M5DT4H3M2S",
+                (false, 7, 6, 5, 4, 3, 2, 0),
+                "P7Y6M5DT4H3M2S",
+            ),
+            ("PT0S", (false, 0, 0, 0, 0, 0, 0, 0), "PT0S"),
+            ("P1Y", (false, 1, 0, 0, 0, 0, 0, 0), "P1Y"),
+            ("P1M", (false, 0, 1, 0, 0, 0, 0, 0), "P1M"),
+            ("P1D", (false, 0, 0, 1, 0, 0, 0, 0), "P1D"),
+            ("PT1H", (false, 0, 0, 0, 1, 0, 0, 0), "PT1H"),
+            ("PT1M", (false, 0, 0, 0, 0, 1, 0, 0), "PT1M"),
+            ("PT1S", (false, 0, 0, 0, 0, 0, 1, 0), "PT1S"),
+            ("PT0.001S", (false, 0, 0, 0, 0, 0, 0, 1), "PT0.001S"),
+        ];
+
+        for (input, (negative, years, months, days, hours, mins, secs, millis), output) in
+            test_values
+        {
+            let duration = Duration::from_str(input).unwrap();
+            assert_eq!(duration.is_negative(), negative);
+            assert_eq!(duration.years(), years);
+            assert_eq!(duration.months(), months);
+            assert_eq!(duration.days(), days);
+            assert_eq!(duration.hours(), hours);
+            assert_eq!(duration.mins(), mins);
+            assert_eq!(duration.secs(), secs);
+            assert_eq!(duration.millis(), millis);
+
+            assert_eq!(duration.to_string(), output);
+        }
+
+        let bad_values = [
+            "P",
+            "-P",
+            "P1H",
+            "P1HT",
+            "P1S",
+            "P1ST",
+            "PT1Y",
+            "PT1D",
+            "PT",
+            "P1YT",
+            "P65536Y",
+            "P4294967296M",
+            "P4294967296D",
+            "PT4294967296H",
+            "PT4294967296M",
+            "P1DT18446744073709551.616S",
+        ];
+        for input in bad_values {
+            let result = Duration::from_str(input);
+            eprintln!("{result:?}");
+            assert!(result.is_err());
+        }
+    }
+}
diff --git a/src/util/xsd/mod.rs b/src/util/xsd/mod.rs
new file mode 100644
index 0000000..80b474b
--- /dev/null
+++ b/src/util/xsd/mod.rs
@@ -0,0 +1,19 @@
+mod duration;
+pub use duration::Duration;
+
+#[derive(Debug)]
+pub struct FromStrError {
+    msg: String,
+}
+
+fn from_str_error<T, O>(msg: O) -> FromStrError
+where
+    T: Into<String>,
+    O: Into<Option<T>>,
+{
+    let msg = msg
+        .into()
+        .map(|t| t.into())
+        .unwrap_or_else(|| String::from("Invalid value"));
+    FromStrError { msg }
+}