diff --git a/ifc/Cargo.lock b/ifc/Cargo.lock
index 2d31440..5e8465a 100644
--- a/ifc/Cargo.lock
+++ b/ifc/Cargo.lock
@@ -171,6 +171,7 @@ dependencies = [
  "params",
  "router",
  "serde",
+ "thiserror",
 ]
 
 [[package]]
@@ -816,6 +817,26 @@ dependencies = [
  "remove_dir_all",
 ]
 
+[[package]]
+name = "thiserror"
+version = "1.0.38"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a9cd18aa97d5c45c6603caea1da6628790b37f7a34b6ca89522331c5180fed0"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "1.0.38"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1fb327af4685e4d03fa8cbcf1716380da910eeb2bb8be417e7f9fd3fb164f36f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "time"
 version = "0.1.44"
diff --git a/ifc/Cargo.toml b/ifc/Cargo.toml
index f9cba62..3e20eee 100644
--- a/ifc/Cargo.toml
+++ b/ifc/Cargo.toml
@@ -8,18 +8,19 @@ keywords = ["date", "time", "calendar"]
 categories = ["date-and-time"]
 
 [dependencies]
-chrono = "0.4"
-chrono-tz = "0.6"
-iron = "0.6.1"
-mustache = "0.9.0"
-params = "*"
-router = "*"
-serde = { version = "1.0", features = ["derive"] }
+chrono =        { version = "0.4" }
+chrono-tz =     { version = "0.6" }
+iron =          { version = "0.6.1" }
+mustache =      { version = "0.9.0" }
+params =        { version = "*" }
+router =        { version = "*" }
+serde =         { version = "1.0", features = ["derive"] }
+thiserror =     { version = "1" }
 
-[[bin]]
-name = "ifc-today"
-path = "src/today.rs"
+# [[bin]]
+# name = "ifc-today"
+# path = "src/today.rs"
 
-[[bin]]
-name = "ifc-web"
-path = "src/web.rs"
+# [[bin]]
+# name = "ifc-web"
+# path = "src/web.rs"
diff --git a/ifc/src/lib.rs b/ifc/src/lib.rs
index f99b294..a2adaeb 100644
--- a/ifc/src/lib.rs
+++ b/ifc/src/lib.rs
@@ -1,7 +1,12 @@
-extern crate chrono;
-extern crate chrono_tz;
-
 use chrono::{Datelike, NaiveDate};
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+
+#[derive(Clone, Debug, Error, PartialEq)]
+pub enum Error {
+    #[error("invalid date")]
+    InvalidDate,
+}
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum DayOfWeek {
@@ -92,6 +97,110 @@ impl From<Month> for String {
     }
 }
 
+fn is_leap_year(year: i32) -> bool {
+    NaiveDate::from_ymd(year, 12, 31).ordinal() == 366
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub enum IFC {
+    LeapDay(i32),
+    YearDay(i32),
+    Day(Day),
+}
+
+impl IFC {
+    fn leap_day(year: i32) -> Result<Self, Error> {
+        if is_leap_year(year) {
+            Ok(Self::LeapDay(year))
+        } else {
+            Err(Error::InvalidDate)
+        }
+    }
+
+    fn year_day(year: i32) -> Self {
+        Self::YearDay(year)
+    }
+
+    fn ymd(year: i32, month: u8, day: u8) -> Result<Self, Error> {
+        if month < 1 || month > 13 {
+            Err(Error::InvalidDate)
+        } else if day < 1 || day > 28 {
+            Err(Error::InvalidDate)
+        } else {
+            Ok(Self::Day(Day { year, month, day }))
+        }
+    }
+
+    fn day_of_week(&self) -> DayOfWeek {
+        match *self {
+            IFC::LeapDay(_) => DayOfWeek::LeapDay,
+            IFC::YearDay(_) => DayOfWeek::YearDay,
+            IFC::Day(Day { day, .. }) => match (day - 1) % 7 {
+                0 => DayOfWeek::Sunday,
+                1 => DayOfWeek::Monday,
+                2 => DayOfWeek::Tuesday,
+                3 => DayOfWeek::Wednesday,
+                4 => DayOfWeek::Thursday,
+                5 => DayOfWeek::Friday,
+                6 => DayOfWeek::Saturday,
+                _ => panic!("impossible calculation"),
+            },
+        }
+    }
+
+    fn day_ordinal(&self) -> u32 {
+        self.day_ordinal0() + 1
+    }
+
+    fn day_ordinal0(&self) -> u32 {
+        match *self {
+            IFC::LeapDay(_) => 168,
+            IFC::YearDay(year) => {
+                if is_leap_year(year) {
+                    365
+                } else {
+                    364
+                }
+            }
+            IFC::Day(Day { year, month, day }) => {
+                u32::from(month - 1) * 28
+                    + u32::from(day - 1)
+                    + if is_leap_year(year) && month > 6 {
+                        1
+                    } else {
+                        0
+                    }
+            }
+        }
+    }
+
+    fn week_ordinal(&self) -> u32 {
+        self.week_ordinal0() + 1
+    }
+
+    fn week_ordinal0(&self) -> u32 {
+        match *self {
+            IFC::LeapDay(_) => 0,
+            IFC::YearDay(_) => 0,
+            IFC::Day(Day { month, day, .. }) => u32::from(month - 1) * 4 + (u32::from(day - 1) / 7),
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Day {
+    year: i32,
+    month: u8,
+    day: u8,
+}
+
+impl From<chrono::NaiveDate> for Day {
+    fn from(date: chrono::NaiveDate) -> Self {
+        unimplemented!();
+    }
+}
+
+/*
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct IFC {
     year: u32,
@@ -99,12 +208,9 @@ pub struct IFC {
     leap_year: bool,
 }
 
-fn is_leap_year(year: i32) -> bool {
-    NaiveDate::from_ymd(year, 12, 31).ordinal() == 366
-}
 
 impl IFC {
-    pub fn ymd(year: u32, month: u8, day: u8) -> IFC {
+    pub fn ymd(year: u32, month: u8, day: u8) -> Self {
         let leap_year = is_leap_year(year as i32 - 10000);
 
         let ordinal = if is_leap_year(year as i32 - 10000) {
@@ -126,7 +232,18 @@ impl IFC {
         }
     }
 
-    pub fn weekday_ifc(&self) -> DayOfWeek {
+    pub fn year_day(year: u32) -> Self {
+        Self {
+            year,
+            ordinal: if is_leap_year(year) { 366 } else { 365 }
+        }
+    }
+
+    pub fn leap_day(year: u32) -> Result<IFC, Error> {
+    }
+
+    pub fn weekday(&self) -> DayOfWeek {
+        // if self.day.ordinal ==
         match self.day() % 7 {
             0 => DayOfWeek::Saturday,
             1 => DayOfWeek::Sunday,
@@ -143,7 +260,9 @@ impl IFC {
         Month::from(self.month())
     }
 }
+*/
 
+/*
 impl From<chrono::Date<chrono::Utc>> for IFC {
     fn from(d: chrono::Date<chrono::Utc>) -> IFC {
         IFC::from(d.naive_utc())
@@ -166,7 +285,9 @@ impl From<chrono::NaiveDate> for IFC {
         }
     }
 }
+*/
 
+/*
 impl Datelike for IFC {
     fn year(&self) -> i32 {
         self.year as i32
@@ -264,6 +385,7 @@ impl Datelike for IFC {
         })
     }
 }
+*/
 
 #[cfg(test)]
 mod tests {
@@ -271,6 +393,188 @@ mod tests {
 
     use chrono::NaiveDate;
 
+    #[test]
+    fn creates_a_day() {
+        assert_eq!(IFC::leap_day(12020), Ok(IFC::LeapDay(12020)));
+        assert_eq!(IFC::year_day(12020), IFC::YearDay(12020));
+        assert_eq!(
+            IFC::ymd(12022, 13, 28),
+            Ok(IFC::Day(Day {
+                year: 12022,
+                month: 13,
+                day: 28
+            }))
+        );
+    }
+
+    #[test]
+    fn rejects_invalid_dates() {
+        assert_eq!(IFC::leap_day(12022), Err(Error::InvalidDate));
+        assert_eq!(IFC::ymd(12022, 13, 0), Err(Error::InvalidDate));
+        assert_eq!(IFC::ymd(12022, 14, 1), Err(Error::InvalidDate));
+    }
+
+    #[test]
+    fn it_expresses_day_of_week() {
+        assert_eq!(
+            IFC::leap_day(12020).unwrap().day_of_week(),
+            DayOfWeek::LeapDay
+        );
+        assert_eq!(IFC::year_day(12022).day_of_week(), DayOfWeek::YearDay);
+        assert_eq!(
+            IFC::ymd(12022, 1, 1).unwrap().day_of_week(),
+            DayOfWeek::Sunday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 2).unwrap().day_of_week(),
+            DayOfWeek::Monday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 3).unwrap().day_of_week(),
+            DayOfWeek::Tuesday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 4).unwrap().day_of_week(),
+            DayOfWeek::Wednesday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 5).unwrap().day_of_week(),
+            DayOfWeek::Thursday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 6).unwrap().day_of_week(),
+            DayOfWeek::Friday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 7).unwrap().day_of_week(),
+            DayOfWeek::Saturday
+        );
+        assert_eq!(
+            IFC::ymd(12022, 1, 8).unwrap().day_of_week(),
+            DayOfWeek::Sunday
+        );
+    }
+
+    #[test]
+    fn it_reports_ordinal_days() {
+        assert_eq!(IFC::ymd(12022, 1, 1).unwrap().day_ordinal(), 1);
+        assert_eq!(IFC::ymd(12022, 1, 1).unwrap().day_ordinal0(), 0);
+
+        assert_eq!(IFC::ymd(12022, 1, 28).unwrap().day_ordinal(), 28);
+        assert_eq!(IFC::ymd(12022, 1, 28).unwrap().day_ordinal0(), 27);
+
+        assert_eq!(IFC::ymd(12022, 2, 1).unwrap().day_ordinal(), 29);
+        assert_eq!(IFC::ymd(12022, 2, 1).unwrap().day_ordinal0(), 28);
+
+        assert_eq!(IFC::ymd(12022, 3, 1).unwrap().day_ordinal(), 57);
+        assert_eq!(IFC::ymd(12022, 3, 1).unwrap().day_ordinal0(), 56);
+
+        assert_eq!(IFC::ymd(12022, 4, 1).unwrap().day_ordinal(), 85);
+        assert_eq!(IFC::ymd(12022, 4, 1).unwrap().day_ordinal0(), 84);
+
+        assert_eq!(IFC::ymd(12022, 5, 1).unwrap().day_ordinal(), 113);
+        assert_eq!(IFC::ymd(12022, 5, 1).unwrap().day_ordinal0(), 112);
+
+        assert_eq!(IFC::ymd(12022, 6, 1).unwrap().day_ordinal(), 141);
+        assert_eq!(IFC::ymd(12022, 6, 1).unwrap().day_ordinal0(), 140);
+
+        assert_eq!(IFC::ymd(12022, 7, 1).unwrap().day_ordinal(), 169);
+        assert_eq!(IFC::ymd(12022, 7, 1).unwrap().day_ordinal0(), 168);
+
+        assert_eq!(IFC::ymd(12022, 8, 1).unwrap().day_ordinal(), 197);
+        assert_eq!(IFC::ymd(12022, 8, 1).unwrap().day_ordinal0(), 196);
+
+        assert_eq!(IFC::ymd(12022, 9, 1).unwrap().day_ordinal(), 225);
+        assert_eq!(IFC::ymd(12022, 9, 1).unwrap().day_ordinal0(), 224);
+
+        assert_eq!(IFC::ymd(12022, 10, 1).unwrap().day_ordinal(), 253);
+        assert_eq!(IFC::ymd(12022, 10, 1).unwrap().day_ordinal0(), 252);
+
+        assert_eq!(IFC::ymd(12022, 11, 1).unwrap().day_ordinal(), 281);
+        assert_eq!(IFC::ymd(12022, 11, 1).unwrap().day_ordinal0(), 280);
+
+        assert_eq!(IFC::ymd(12022, 12, 1).unwrap().day_ordinal(), 309);
+        assert_eq!(IFC::ymd(12022, 12, 1).unwrap().day_ordinal0(), 308);
+
+        assert_eq!(IFC::ymd(12022, 13, 1).unwrap().day_ordinal(), 337);
+        assert_eq!(IFC::ymd(12022, 13, 1).unwrap().day_ordinal0(), 336);
+
+        assert_eq!(IFC::ymd(12022, 13, 28).unwrap().day_ordinal(), 364);
+        assert_eq!(IFC::ymd(12022, 13, 28).unwrap().day_ordinal0(), 363);
+
+        assert_eq!(IFC::year_day(12022).day_ordinal(), 365);
+    }
+
+    #[test]
+    fn it_reports_ordinal_days_on_leap_year() {
+        assert_eq!(IFC::ymd(12020, 1, 1).unwrap().day_ordinal(), 1);
+        assert_eq!(IFC::ymd(12020, 1, 1).unwrap().day_ordinal0(), 0);
+
+        assert_eq!(IFC::ymd(12020, 1, 28).unwrap().day_ordinal(), 28);
+        assert_eq!(IFC::ymd(12020, 1, 28).unwrap().day_ordinal0(), 27);
+
+        assert_eq!(IFC::ymd(12020, 2, 1).unwrap().day_ordinal(), 29);
+        assert_eq!(IFC::ymd(12020, 2, 1).unwrap().day_ordinal0(), 28);
+
+        assert_eq!(IFC::ymd(12020, 3, 1).unwrap().day_ordinal(), 57);
+        assert_eq!(IFC::ymd(12020, 3, 1).unwrap().day_ordinal0(), 56);
+
+        assert_eq!(IFC::ymd(12020, 4, 1).unwrap().day_ordinal(), 85);
+        assert_eq!(IFC::ymd(12020, 4, 1).unwrap().day_ordinal0(), 84);
+
+        assert_eq!(IFC::ymd(12020, 5, 1).unwrap().day_ordinal(), 113);
+        assert_eq!(IFC::ymd(12020, 5, 1).unwrap().day_ordinal0(), 112);
+
+        assert_eq!(IFC::ymd(12020, 6, 1).unwrap().day_ordinal(), 141);
+        assert_eq!(IFC::ymd(12020, 6, 1).unwrap().day_ordinal0(), 140);
+
+        assert_eq!(IFC::leap_day(12020).unwrap().day_ordinal(), 169);
+        assert_eq!(IFC::leap_day(12020).unwrap().day_ordinal0(), 168);
+
+        assert_eq!(IFC::ymd(12020, 7, 1).unwrap().day_ordinal(), 170);
+        assert_eq!(IFC::ymd(12020, 7, 1).unwrap().day_ordinal0(), 169);
+
+        assert_eq!(IFC::ymd(12020, 8, 1).unwrap().day_ordinal(), 198);
+        assert_eq!(IFC::ymd(12020, 8, 1).unwrap().day_ordinal0(), 197);
+
+        assert_eq!(IFC::ymd(12020, 9, 1).unwrap().day_ordinal(), 226);
+        assert_eq!(IFC::ymd(12020, 9, 1).unwrap().day_ordinal0(), 225);
+
+        assert_eq!(IFC::ymd(12020, 10, 1).unwrap().day_ordinal(), 254);
+        assert_eq!(IFC::ymd(12020, 10, 1).unwrap().day_ordinal0(), 253);
+
+        assert_eq!(IFC::ymd(12020, 11, 1).unwrap().day_ordinal(), 282);
+        assert_eq!(IFC::ymd(12020, 11, 1).unwrap().day_ordinal0(), 281);
+
+        assert_eq!(IFC::ymd(12020, 12, 1).unwrap().day_ordinal(), 310);
+        assert_eq!(IFC::ymd(12020, 12, 1).unwrap().day_ordinal0(), 309);
+
+        assert_eq!(IFC::ymd(12020, 13, 1).unwrap().day_ordinal(), 338);
+        assert_eq!(IFC::ymd(12020, 13, 1).unwrap().day_ordinal0(), 337);
+
+        assert_eq!(IFC::ymd(12020, 13, 28).unwrap().day_ordinal(), 365);
+        assert_eq!(IFC::ymd(12020, 13, 28).unwrap().day_ordinal0(), 364);
+
+        assert_eq!(IFC::year_day(12020).day_ordinal(), 366);
+    }
+
+    #[test]
+    fn it_reports_ordinal_weeks() {
+        assert_eq!(IFC::ymd(12022, 1, 1).unwrap().week_ordinal(), 1);
+        assert_eq!(IFC::ymd(12022, 1, 1).unwrap().week_ordinal0(), 0);
+        assert_eq!(IFC::ymd(12022, 1, 4).unwrap().week_ordinal(), 1);
+        assert_eq!(IFC::ymd(12022, 1, 4).unwrap().week_ordinal0(), 0);
+        assert_eq!(IFC::ymd(12022, 1, 7).unwrap().week_ordinal(), 1);
+        assert_eq!(IFC::ymd(12022, 1, 7).unwrap().week_ordinal0(), 0);
+        assert_eq!(IFC::ymd(12022, 1, 8).unwrap().week_ordinal(), 2);
+        assert_eq!(IFC::ymd(12022, 1, 8).unwrap().week_ordinal0(), 1);
+        assert_eq!(IFC::ymd(12022, 2, 1).unwrap().week_ordinal(), 5);
+        assert_eq!(IFC::ymd(12022, 2, 1).unwrap().week_ordinal0(), 4);
+        assert_eq!(IFC::ymd(12022, 13, 28).unwrap().week_ordinal(), 52);
+        assert_eq!(IFC::ymd(12022, 13, 28).unwrap().week_ordinal0(), 51);
+    }
+
+    /*
     #[test]
     fn check_start_of_month() {
         assert_eq!(
@@ -379,6 +683,9 @@ mod tests {
         );
     }
 
+    #[test]
+    fn report_leap_day() {}
+
     #[test]
     fn check_start_of_month_leap_year() {
         assert_eq!(
@@ -651,20 +958,25 @@ mod tests {
         assert_eq!(IFC::ymd(12019, 13, 29).day(), 29);
         assert_eq!(IFC::ymd(12019, 13, 29).day0(), 28);
     }
+    */
 
+    /*
     #[test]
     fn it_reports_correct_day_of_week() {
-        assert_eq!(IFC::ymd(12019, 1, 1).weekday(), chrono::Weekday::Sun);
-        assert_eq!(IFC::ymd(12019, 6, 1).weekday(), chrono::Weekday::Sun);
-        assert_eq!(IFC::ymd(12019, 6, 28).weekday(), chrono::Weekday::Sat);
-        assert_eq!(IFC::ymd(12019, 7, 1).weekday(), chrono::Weekday::Sun);
-        assert_eq!(IFC::ymd(12019, 13, 28).weekday(), chrono::Weekday::Sat);
-        assert_eq!(IFC::ymd(12019, 13, 29).weekday(), chrono::Weekday::Sat);
+        assert_eq!(IFC::ymd(12019, 1, 1).weekday(), DayOfWeek::Sunday);
+        assert_eq!(IFC::ymd(12019, 6, 1).weekday(), DayOfWeek::Sunday);
+        assert_eq!(IFC::ymd(12019, 6, 28).weekday(), DayOfWeek::Saturday);
+        assert_eq!(IFC::ymd(12019, 7, 1).weekday(), DayOfWeek::Sunday);
+        assert_eq!(IFC::ymd(12019, 13, 28).weekday(), DayOfWeek::Saturday);
+        assert_eq!(IFC::ymd(12019, 13, 29).weekday(), DayOfWeek::Saturday);
 
-        assert_eq!(IFC::ymd(12020, 6, 28).weekday(), chrono::Weekday::Sat);
-        assert_eq!(IFC::ymd(12020, 6, 29).weekday(), chrono::Weekday::Sat);
-        assert_eq!(IFC::ymd(12020, 7, 1).weekday(), chrono::Weekday::Sun);
-        assert_eq!(IFC::ymd(12020, 13, 28).weekday(), chrono::Weekday::Sat);
-        assert_eq!(IFC::ymd(12020, 13, 29).weekday(), chrono::Weekday::Sat);
+        assert_eq!(IFC::ymd(12020, 6, 28).weekday(), DayOfWeek::Saturday);
+        assert_eq!(IFC::ymd(12020, 6, 29).weekday(), DayOfWeek::Saturday);
+        assert_eq!(IFC::ymd(12020, 7, 1).weekday(), DayOfWeek::Sunday);
+        assert_eq!(IFC::ymd(12020, 13, 28).weekday(), DayOfWeek::Saturday);
+        assert_eq!(IFC::ymd(12020, 13, 29).weekday(), DayOfWeek::Saturday);
+
+        assert_eq!(IFC::ymd(12022, 13, 31)
     }
+    */
 }