diff --git a/Cargo.toml b/Cargo.toml
index 5185872..7893ecb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,17 +24,17 @@ appveyor = { repository = "chronotope/chrono" }
 name = "chrono"
 
 [features]
-default = ["clock"]
-clock = ["time"]
+default = ["clock", "std"]
+std = []
+clock = ["time", "std"]
 wasmbind = ["wasm-bindgen", "js-sys"]
 
 [dependencies]
-libc = { version = "0.2", default-features = false }
 time = { version = "0.1.39", optional = true }
 num-integer = { version = "0.1.36", default-features = false }
 num-traits = { version = "0.2", default-features = false }
 rustc-serialize = { version = "0.3.20", optional = true }
-serde = { version = "1", optional = true }
+serde = { version = "1.0.99", default-features = false, optional = true }
 
 [target.'cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))'.dependencies]
 wasm-bindgen = { version = "0.2", optional = true }
diff --git a/ci/core-test/Cargo.toml b/ci/core-test/Cargo.toml
new file mode 100644
index 0000000..ad75e16
--- /dev/null
+++ b/ci/core-test/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "core-test"
+version = "0.1.0"
+authors = [
+    "Kang Seonghoon <public+rust@mearie.org>",
+    "Brandon W Maister <quodlibetor@gmail.com>",
+]
+edition = "2018"
+
+[dependencies]
+chrono = { path = "../..", default-features = false, features = ["serde"] }
diff --git a/ci/core-test/src/lib.rs b/ci/core-test/src/lib.rs
new file mode 100644
index 0000000..0c9ac1a
--- /dev/null
+++ b/ci/core-test/src/lib.rs
@@ -0,0 +1 @@
+#![no_std]
diff --git a/ci/travis.sh b/ci/travis.sh
index 98e25ce..809f234 100755
--- a/ci/travis.sh
+++ b/ci/travis.sh
@@ -85,6 +85,13 @@ build_only() {
   channel build -v --no-default-features
 }
 
+build_core_test() {
+    rustup target add thumbv6m-none-eabi --toolchain $CHANNEL
+    cd ci/core-test
+    channel build -v --target thumbv6m-none-eabi
+    cd ../..
+}
+
 run_clippy() {
     # cached installation will not work on a later nightly
     if [ -n "${TRAVIS}" ] && ! cargo install clippy --debug --force; then
@@ -119,3 +126,6 @@ build_and_test
 
 CHANNEL=1.13.0
 build_only
+
+CHANNEL=stable
+build_core_test
diff --git a/src/date.rs b/src/date.rs
index f8b59ce..a6e5ab8 100644
--- a/src/date.rs
+++ b/src/date.rs
@@ -3,9 +3,9 @@
 
 //! ISO 8601 calendar date with time zone.
 
-use std::{fmt, hash};
-use std::cmp::Ordering;
-use std::ops::{Add, Sub};
+use core::{fmt, hash};
+use core::cmp::Ordering;
+use core::ops::{Add, Sub};
 use oldtime::Duration as OldDuration;
 
 use {Weekday, Datelike};
diff --git a/src/datetime.rs b/src/datetime.rs
index b4881bf..75a6922 100644
--- a/src/datetime.rs
+++ b/src/datetime.rs
@@ -3,12 +3,16 @@
 
 //! ISO 8601 date and time with time zone.
 
-use std::{str, fmt, hash};
-use std::cmp::Ordering;
-use std::ops::{Add, Sub};
+use core::{str, fmt, hash};
+use core::cmp::Ordering;
+use core::ops::{Add, Sub};
+#[cfg(any(feature = "std", test))]
 use std::time::{SystemTime, UNIX_EPOCH};
 use oldtime::Duration as OldDuration;
 
+#[cfg(not(any(feature = "std", test)))]
+use alloc::string::{String, ToString};
+
 use {Weekday, Timelike, Datelike};
 #[cfg(feature="clock")]
 use offset::Local;
@@ -647,6 +651,7 @@ impl str::FromStr for DateTime<Local> {
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl From<SystemTime> for DateTime<Utc> {
     fn from(t: SystemTime) -> DateTime<Utc> {
         let (sec, nsec) = match t.duration_since(UNIX_EPOCH) {
@@ -672,6 +677,7 @@ impl From<SystemTime> for DateTime<Local> {
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl<Tz: TimeZone> From<DateTime<Tz>> for SystemTime {
     fn from(dt: DateTime<Tz>) -> SystemTime {
         use std::time::Duration;
@@ -699,7 +705,7 @@ fn test_auto_conversion() {
 fn test_encodable_json<FUtc, FFixed, E>(to_string_utc: FUtc, to_string_fixed: FFixed)
     where FUtc: Fn(&DateTime<Utc>) -> Result<String, E>,
           FFixed: Fn(&DateTime<FixedOffset>) -> Result<String, E>,
-          E: ::std::fmt::Debug
+          E: ::core::fmt::Debug
 {
     assert_eq!(to_string_utc(&Utc.ymd(2014, 7, 24).and_hms(12, 34, 6)).ok(),
                Some(r#""2014-07-24T12:34:06Z""#.into()));
@@ -717,7 +723,7 @@ fn test_decodable_json<FUtc, FFixed, FLocal, E>(utc_from_str: FUtc,
     where FUtc: Fn(&str) -> Result<DateTime<Utc>, E>,
           FFixed: Fn(&str) -> Result<DateTime<FixedOffset>, E>,
           FLocal: Fn(&str) -> Result<DateTime<Local>, E>,
-          E: ::std::fmt::Debug
+          E: ::core::fmt::Debug
 {
     // should check against the offset as well (the normal DateTime comparison will ignore them)
     fn norm<Tz: TimeZone>(dt: &Option<DateTime<Tz>>) -> Option<(&DateTime<Tz>, &Tz::Offset)> {
@@ -754,7 +760,7 @@ fn test_decodable_json_timestamps<FUtc, FFixed, FLocal, E>(utc_from_str: FUtc,
     where FUtc: Fn(&str) -> Result<rustc_serialize::TsSeconds<Utc>, E>,
           FFixed: Fn(&str) -> Result<rustc_serialize::TsSeconds<FixedOffset>, E>,
           FLocal: Fn(&str) -> Result<rustc_serialize::TsSeconds<Local>, E>,
-          E: ::std::fmt::Debug
+          E: ::core::fmt::Debug
 {
     fn norm<Tz: TimeZone>(dt: &Option<DateTime<Tz>>) -> Option<(&DateTime<Tz>, &Tz::Offset)> {
         dt.as_ref().map(|dt| (dt, dt.offset()))
@@ -778,8 +784,8 @@ fn test_decodable_json_timestamps<FUtc, FFixed, FLocal, E>(utc_from_str: FUtc,
 
 #[cfg(feature = "rustc-serialize")]
 pub mod rustc_serialize {
-    use std::fmt;
-    use std::ops::Deref;
+    use core::fmt;
+    use core::ops::Deref;
     use super::DateTime;
     #[cfg(feature="clock")]
     use offset::Local;
@@ -907,7 +913,9 @@ pub mod rustc_serialize {
 /// documented at re-export site
 #[cfg(feature = "serde")]
 pub mod serde {
-    use std::fmt;
+    use core::fmt;
+    #[cfg(not(any(feature = "std", test)))]
+    use alloc::format;
     use super::DateTime;
     #[cfg(feature="clock")]
     use offset::Local;
@@ -967,7 +975,7 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_nanoseconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
         use {DateTime, Utc};
@@ -1114,7 +1122,7 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_milliseconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
         use {DateTime, Utc};
@@ -1261,7 +1269,7 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_seconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
         use {DateTime, Utc};
diff --git a/src/format/mod.rs b/src/format/mod.rs
index 24647b4..e4e6776 100644
--- a/src/format/mod.rs
+++ b/src/format/mod.rs
@@ -17,9 +17,13 @@
 
 #![allow(ellipsis_inclusive_range_patterns)]
 
-use std::fmt;
-use std::str::FromStr;
+use core::fmt;
+use core::str::FromStr;
+#[cfg(any(feature = "std", test))]
 use std::error::Error;
+use alloc::boxed::Box;
+#[cfg(not(any(feature = "std", test)))]
+use alloc::string::{String, ToString};
 
 use {Datelike, Timelike, Weekday, ParseWeekdayError};
 use div::{div_floor, mod_floor};
@@ -30,7 +34,7 @@ pub use self::strftime::StrftimeItems;
 pub use self::parsed::Parsed;
 pub use self::parse::parse;
 
-/// An unhabitated type used for `InternalNumeric` and `InternalFixed` below.
+/// An uninhabited type used for `InternalNumeric` and `InternalFixed` below.
 #[derive(Clone, PartialEq, Eq)]
 enum Void {}
 
@@ -55,7 +59,7 @@ pub enum Pad {
 ///
 /// The **parsing width** is the maximal width to be scanned.
 /// The parser only tries to consume from one to given number of digits (greedily).
-/// It also trims the preceding whitespaces if any.
+/// It also trims the preceding whitespace if any.
 /// It cannot parse the negative number, so some date and time cannot be formatted then
 /// parsed with the same formatting items.
 #[derive(Clone, PartialEq, Eq, Debug)]
@@ -185,13 +189,13 @@ pub enum Fixed {
     TimezoneName,
     /// Offset from the local time to UTC (`+09:00` or `-04:00` or `+00:00`).
     ///
-    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespaces.
+    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespace.
     /// The offset is limited from `-24:00` to `+24:00`,
     /// which is same to [`FixedOffset`](../offset/struct.FixedOffset.html)'s range.
     TimezoneOffsetColon,
     /// Offset from the local time to UTC (`+09:00` or `-04:00` or `Z`).
     ///
-    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespaces,
+    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespace,
     /// and `Z` can be either in upper case or in lower case.
     /// The offset is limited from `-24:00` to `+24:00`,
     /// which is same to [`FixedOffset`](../offset/struct.FixedOffset.html)'s range.
@@ -305,13 +309,7 @@ enum ParseErrorKind {
 /// Same to `Result<T, ParseError>`.
 pub type ParseResult<T> = Result<T, ParseError>;
 
-impl fmt::Display for ParseError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.description().fmt(f)
-    }
-}
-
-impl Error for ParseError {
+impl ParseError {
     fn description(&self) -> &str {
         match self.0 {
             ParseErrorKind::OutOfRange => "input is out of range",
@@ -325,6 +323,19 @@ impl Error for ParseError {
     }
 }
 
+impl fmt::Display for ParseError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.description().fmt(f)
+    }
+}
+
+#[cfg(any(feature = "std", test))]
+impl Error for ParseError {
+    fn description(&self) -> &str {
+        self.description()
+    }
+}
+
 // to be used in this module and submodules
 const OUT_OF_RANGE: ParseError = ParseError(ParseErrorKind::OutOfRange);
 const IMPOSSIBLE:   ParseError = ParseError(ParseErrorKind::Impossible);
@@ -356,7 +367,7 @@ pub fn format<'a, I>(
     static LONG_WEEKDAYS: [&'static str; 7] =
         ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"];
 
-    use std::fmt::Write;
+    use core::fmt::Write;
     let mut result = String::new();
 
     for item in items {
diff --git a/src/format/parse.rs b/src/format/parse.rs
index 01e326e..c10a802 100644
--- a/src/format/parse.rs
+++ b/src/format/parse.rs
@@ -6,7 +6,7 @@
 
 #![allow(deprecated)]
 
-use std::usize;
+use core::usize;
 
 use Weekday;
 
diff --git a/src/lib.rs b/src/lib.rs
index b43fa0d..704f3b0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -387,6 +387,8 @@
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
 
+#![cfg_attr(not(any(feature = "std", test)), no_std)]
+
 // The explicit 'static lifetimes are still needed for rustc 1.13-16
 // backward compatibility, and this appeases clippy. If minimum rustc
 // becomes 1.17, should be able to remove this, those 'static lifetimes,
@@ -403,6 +405,13 @@
     trivially_copy_pass_by_ref,
 ))]
 
+#[cfg(not(any(feature = "std", test)))]
+extern crate alloc;
+#[cfg(any(feature = "std", test))]
+extern crate std as core;
+#[cfg(any(feature = "std", test))]
+extern crate std as alloc;
+
 #[cfg(feature="clock")]
 extern crate time as oldtime;
 extern crate num_integer;
@@ -678,7 +687,7 @@ impl num_traits::FromPrimitive for Weekday {
     }
 }
 
-use std::fmt;
+use core::fmt;
 
 /// An error resulting from reading `Weekday` value with `FromStr`.
 #[derive(Clone, PartialEq)]
@@ -697,7 +706,9 @@ impl fmt::Debug for ParseWeekdayError {
 #[cfg(feature = "serde")]
 mod weekday_serde {
     use super::Weekday;
-    use std::fmt;
+    use core::fmt;
+    #[cfg(not(any(feature = "std", test)))]
+    use alloc::format;
     use serdelib::{ser, de};
 
     impl ser::Serialize for Weekday {
diff --git a/src/naive/date.rs b/src/naive/date.rs
index aad98bc..f429597 100644
--- a/src/naive/date.rs
+++ b/src/naive/date.rs
@@ -3,8 +3,8 @@
 
 //! ISO 8601 calendar date without timezone.
 
-use std::{str, fmt};
-use std::ops::{Add, Sub, AddAssign, SubAssign};
+use core::{str, fmt};
+use core::ops::{Add, Sub, AddAssign, SubAssign};
 use num_traits::ToPrimitive;
 use oldtime::Duration as OldDuration;
 
@@ -1387,7 +1387,7 @@ impl SubAssign<OldDuration> for NaiveDate {
 
 /// Subtracts another `NaiveDate` from the current date.
 /// Returns a `Duration` of integral numbers.
-/// 
+///
 /// This does not overflow or underflow at all,
 /// as all possible output fits in the range of `Duration`.
 ///
@@ -1600,7 +1600,9 @@ mod rustc_serialize {
 
 #[cfg(feature = "serde")]
 mod serde {
-    use std::fmt;
+    use core::fmt;
+    #[cfg(not(any(feature = "std", test)))]
+    use alloc::format;
     use super::NaiveDate;
     use serdelib::{ser, de};
 
@@ -1629,7 +1631,7 @@ mod serde {
     impl<'de> de::Visitor<'de> for NaiveDateVisitor {
         type Value = NaiveDate;
 
-        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result 
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result
         {
             write!(formatter, "a formatted date string")
         }
diff --git a/src/naive/datetime.rs b/src/naive/datetime.rs
index bb9ff8d..6f4fbe7 100644
--- a/src/naive/datetime.rs
+++ b/src/naive/datetime.rs
@@ -3,8 +3,8 @@
 
 //! ISO 8601 date and time without timezone.
 
-use std::{str, fmt, hash};
-use std::ops::{Add, Sub, AddAssign, SubAssign};
+use core::{str, fmt, hash};
+use core::ops::{Add, Sub, AddAssign, SubAssign};
 use num_traits::ToPrimitive;
 use oldtime::Duration as OldDuration;
 
@@ -1663,7 +1663,9 @@ pub mod rustc_serialize {
 /// Tools to help serializing/deserializing `NaiveDateTime`s
 #[cfg(feature = "serde")]
 pub mod serde {
-    use std::fmt;
+    use core::fmt;
+    #[cfg(not(any(feature = "std", test)))]
+    use alloc::format;
     use super::{NaiveDateTime};
     use serdelib::{ser, de};
 
@@ -1750,7 +1752,9 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_nanoseconds {
-        use std::fmt;
+        use core::fmt;
+        #[cfg(not(any(feature = "std", test)))]
+        use alloc::format;
         use serdelib::{ser, de};
 
         use NaiveDateTime;
@@ -1895,7 +1899,9 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_milliseconds {
-        use std::fmt;
+        use core::fmt;
+        #[cfg(not(any(feature = "std", test)))]
+        use alloc::format;
         use serdelib::{ser, de};
 
         use NaiveDateTime;
@@ -2040,7 +2046,9 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_seconds {
-        use std::fmt;
+        use core::fmt;
+        #[cfg(not(any(feature = "std", test)))]
+        use alloc::format;
         use serdelib::{ser, de};
 
         use NaiveDateTime;
diff --git a/src/naive/internals.rs b/src/naive/internals.rs
index dd9d535..a1cd1f6 100644
--- a/src/naive/internals.rs
+++ b/src/naive/internals.rs
@@ -15,7 +15,7 @@
 
 #![allow(dead_code)] // some internal methods have been left for consistency
 
-use std::{i32, fmt};
+use core::{i32, fmt};
 use num_traits::FromPrimitive;
 use Weekday;
 use div::{div_rem, mod_floor};
diff --git a/src/naive/isoweek.rs b/src/naive/isoweek.rs
index 667cf2f..0aeedb0 100644
--- a/src/naive/isoweek.rs
+++ b/src/naive/isoweek.rs
@@ -3,7 +3,7 @@
 
 //! ISO 8601 week.
 
-use std::fmt;
+use core::fmt;
 
 use super::internals::{DateImpl, Of, YearFlags};
 
diff --git a/src/naive/time.rs b/src/naive/time.rs
index 440c8a7..c1cf297 100644
--- a/src/naive/time.rs
+++ b/src/naive/time.rs
@@ -3,8 +3,8 @@
 
 //! ISO 8601 time without timezone.
 
-use std::{str, fmt, hash};
-use std::ops::{Add, Sub, AddAssign, SubAssign};
+use core::{str, fmt, hash};
+use core::ops::{Add, Sub, AddAssign, SubAssign};
 use oldtime::Duration as OldDuration;
 
 use Timelike;
@@ -681,7 +681,7 @@ impl NaiveTime {
         //      `rhs.frac`|========================================>|
         //          |     |   |        `self - rhs`         |       |
 
-        use std::cmp::Ordering;
+        use core::cmp::Ordering;
 
         let secs = i64::from(self.secs) - i64::from(rhs.secs);
         let frac = i64::from(self.frac) - i64::from(rhs.frac);
@@ -1411,7 +1411,9 @@ mod rustc_serialize {
 
 #[cfg(feature = "serde")]
 mod serde {
-    use std::fmt;
+    use core::fmt;
+    #[cfg(not(any(feature = "std", test)))]
+    use alloc::format;
     use super::NaiveTime;
     use serdelib::{ser, de};
 
@@ -1431,7 +1433,7 @@ mod serde {
     impl<'de> de::Visitor<'de> for NaiveTimeVisitor {
         type Value = NaiveTime;
 
-        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result 
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result
         {
             write!(formatter, "a formatted time string")
         }
diff --git a/src/offset/fixed.rs b/src/offset/fixed.rs
index 5b69ef0..01512c0 100644
--- a/src/offset/fixed.rs
+++ b/src/offset/fixed.rs
@@ -3,8 +3,8 @@
 
 //! The time zone which has a fixed offset from UTC.
 
-use std::ops::{Add, Sub};
-use std::fmt;
+use core::ops::{Add, Sub};
+use core::fmt;
 use oldtime::Duration as OldDuration;
 
 use Timelike;
diff --git a/src/offset/mod.rs b/src/offset/mod.rs
index 566e427..a255292 100644
--- a/src/offset/mod.rs
+++ b/src/offset/mod.rs
@@ -18,7 +18,7 @@
 //! and provides implementations for 1 and 3.
 //! An `TimeZone` instance can be reconstructed from the corresponding `Offset` instance.
 
-use std::fmt;
+use core::fmt;
 
 use format::{parse, ParseResult, Parsed, StrftimeItems};
 use naive::{NaiveDate, NaiveDateTime, NaiveTime};
diff --git a/src/offset/utc.rs b/src/offset/utc.rs
index 87a4fba..da8de11 100644
--- a/src/offset/utc.rs
+++ b/src/offset/utc.rs
@@ -3,7 +3,7 @@
 
 //! The UTC (Coordinated Universal Time) time zone.
 
-use std::fmt;
+use core::fmt;
 #[cfg(all(feature="clock", not(all(target_arch = "wasm32", feature = "wasmbind"))))]
 use oldtime;
 
diff --git a/src/oldtime.rs b/src/oldtime.rs
index 2bdc2f6..d523b54 100644
--- a/src/oldtime.rs
+++ b/src/oldtime.rs
@@ -10,10 +10,11 @@
 
 //! Temporal quantification
 
-use std::{fmt, i64};
+use core::{fmt, i64};
+#[cfg(any(feature = "std", test))]
 use std::error::Error;
-use std::ops::{Add, Sub, Mul, Div, Neg};
-use std::time::Duration as StdDuration;
+use core::ops::{Add, Sub, Mul, Div, Neg};
+use core::time::Duration as StdDuration;
 
 /// The number of nanoseconds in a microsecond.
 const NANOS_PER_MICRO: i32 = 1000;
@@ -392,15 +393,22 @@ impl fmt::Display for Duration {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct OutOfRangeError(());
 
+impl OutOfRangeError {
+    fn description(&self) -> &str {
+        "Source duration value is out of range for the target type"
+    }
+}
+
 impl fmt::Display for OutOfRangeError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{}", self.description())
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl Error for OutOfRangeError {
     fn description(&self) -> &str {
-        "Source duration value is out of range for the target type"
+        self.description()
     }
 }
 
diff --git a/src/round.rs b/src/round.rs
index bf62762..ac5b984 100644
--- a/src/round.rs
+++ b/src/round.rs
@@ -2,7 +2,7 @@
 // See README.md and LICENSE.txt for details.
 
 use Timelike;
-use std::ops::{Add, Sub};
+use core::ops::{Add, Sub};
 use oldtime::Duration;
 
 /// Extension trait for subsecond rounding or truncation to a maximum number