2 files changed, 110 insertions, 0 deletions

diff --git a/src/date.rs b/src/date.rs
index 194fd02..43355ab 100644
--- a/src/date.rs
+++ b/src/date.rs
@@ -3,6 +3,8 @@ use crate::{Month, Year};
 use core::cmp::Ordering;
 use core::fmt::Display;
 
+use thiserror::Error;
+
 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
 pub struct Date {
 	year: Year,
@@ -10,6 +12,26 @@ pub struct Date {
 	day: u8,
 }
 
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, Error)]
+#[error("Tried to construct {given_day} {month}, but {month} only has {month_max_day} days")]
+pub struct DayGreaterThanMaximumForMonthError {
+	month: Month,
+	given_day: u8,
+	month_max_day: u8,
+}
+
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, Error)]
+#[error("Tried to construct a leap day in {0} which is not a leap year")]
+pub struct LeapDayNotInLeapYearError(Year);
+
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, Error)]
+pub enum InvalidDateError {
+	#[error("{0}")]
+	DayTooBig(DayGreaterThanMaximumForMonthError),
+	#[error("{0}")]
+	NonLeapYear(LeapDayNotInLeapYearError),
+}
+
 impl Date {
 	/// The earliest date which can be represented
 	pub const MIN: Self = unsafe { Self::from_ymd_unchecked(Year::MIN, Month::January, 1) };
@@ -56,6 +78,42 @@ impl Date {
 		self.year.is_leap_year()
 	}
 
+	// TODO overflow handling
+	pub const fn add_years(self, years: i16) -> Result<Self, LeapDayNotInLeapYearError> {
+		let year = self.year + years;
+
+		if self.day == 29 && self.month == Month::February && !year.is_leap_year() {
+			Err(LeapDayNotInLeapYearError(self.year))
+		} else {
+			Ok(Self {
+				year,
+				month: self.month,
+				day: self.day,
+			})
+		}
+	}
+
+	// TODO overflow handling
+	pub const fn add_months(self, months: i8) -> Result<Self, DayGreaterThanMaximumForMonthError> {
+		let (month, years_to_add) = self.month.add_overflowing(months);
+		let year = self.year + years_to_add;
+		let max_days_for_month = month.days(year.is_leap_year());
+
+		if self.day > max_days_for_month {
+			Err(DayGreaterThanMaximumForMonthError {
+				month,
+				given_day: self.day,
+				month_max_day: max_days_for_month,
+			})
+		} else {
+			Ok(Self {
+				year,
+				month,
+				day: self.day,
+			})
+		}
+	}
+
 	// TODO handle BCE properly
 	pub const fn days_after_common_era(self) -> isize {
 		let year = self.year.wrapping_sub(1);
diff --git a/src/month.rs b/src/month.rs
index fa19324..c5407f7 100644
--- a/src/month.rs
+++ b/src/month.rs
@@ -181,6 +181,7 @@ impl Month {
 
 	// TODO docs
 
+	// TODO handle ordinals greater than 365
 	pub const fn from_ordinal_common(ordinal: u16) -> Self {
 		if ordinal < 31 {
 			January
@@ -314,6 +315,48 @@ impl Month {
 		}
 	}
 
+	pub const fn days_common_year(self) -> u8 {
+		match self {
+			January => 31,
+			February => 28,
+			March => 31,
+			April => 30,
+			May => 31,
+			June => 30,
+			July => 31,
+			August => 31,
+			September => 30,
+			October => 31,
+			November => 30,
+			December => 31,
+		}
+	}
+
+	pub const fn days_leap_year(self) -> u8 {
+		match self {
+			January => 31,
+			February => 29,
+			March => 31,
+			April => 30,
+			May => 31,
+			June => 30,
+			July => 31,
+			August => 31,
+			September => 30,
+			October => 31,
+			November => 30,
+			December => 31,
+		}
+	}
+
+	pub const fn days(self, leap_year: bool) -> u8 {
+		if leap_year {
+			self.days_leap_year()
+		} else {
+			self.days_common_year()
+		}
+	}
+
 	/// Returns the number of days up to the end of the month in a leap year.
 	pub const fn last_day_ordinal_leap(self) -> u16 {
 		match self {
@@ -341,6 +384,15 @@ impl Month {
 			self.last_day_ordinal_common()
 		}
 	}
+
+	pub const fn add_overflowing(self, months: u8) -> (Self, u8) {
+		let zero_indexed_num = ((self as u16) - 1) + months as u16;
+		let wraps = (zero_indexed_num as u8) / 12;
+		let zero_indexed_month = zero_indexed_num % 12;
+		let month = Self::from_u8((zero_indexed_month as u8) + 1).unwrap();
+
+		(month, wraps)
+	}
 }
 
 impl From<Month> for u8 {