Merge pull request #30 from yyankowski/fix-leap-year-add-subtract

fix operations with leap year
yyankowski · Oct 3, 2020 · 803f265 · 803f265
2 parents b2abd4b + 01e20cc
commit 803f265
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Showing 2 changed files with 47 additions and 39 deletions.
diff --git a/src/index.ts b/src/index.ts
@@ -96,19 +96,21 @@ const determineLastDayOfMonth = (year: number, month: number): number => {
 
 const subtractYears = (value: number) => (sDt: Readonly<SimplyDate>): SimplyDate => {
 
-	let day = sDt.day;
+	let { day, month } = sDt;
 	const year = sDt.year - value;
-	// if we are in the zone of days that may be bigger that the total days in the given month
-	// then take the last day of the given month
+
+	// when the source year is a leap year and the target is not
+	// i.e. February 29, 2020 minus 1 year should equal March 1, 2019
 	if (day > 28 && sDt.month === 2) {
 		if (isLeapYear(sDt.year) && !isLeapYear(year)) {
-			day = 28;
+			day = 1;
+			month = 3;
 		}
 	}
 
 	return {
 		year: sDt.year - value,
-		month: sDt.month,
+		month,
 		day,
 		hour: sDt.hour,
 		minute: sDt.minute,
@@ -122,7 +124,7 @@ const subtractYears = (value: number) => (sDt: Readonly<SimplyDate>): SimplyDate
  * @param {number} value
  * @returns {(sDt: SimplyDate) => SimplyDate}
  */
-const  subtractMonths = (value: number) => (sDt: Readonly<SimplyDate>): SimplyDate => {
+const subtractMonths = (value: number) => (sDt: Readonly<SimplyDate>): SimplyDate => {
 	if (value < 0) {
 		return addMonths(-value)(sDt);
 	}
@@ -178,17 +180,19 @@ const addYears = (value: number) => (sDt: Readonly<SimplyDate>): SimplyDate => {
 		return subtractYears(value)(sDt);
 	}
 
-	let day = sDt.day;
+	let { day, month } = sDt;
 	const year = sDt.year + value;
-	if (sDt.month === 2 && day > 28) {
+	// special case from leap to a normal year
+	if (sDt.month === 2 && sDt.day > 28) {
 		if (isLeapYear(sDt.year) && !isLeapYear(year)) {
-			day = 28;
+			day = 1;
+			month = 3;
 		}
 	}
 
 	return {
 		year,
-		month: sDt.month,
+		month,
 		day,
 		hour: sDt.hour,
 		minute: sDt.minute,

diff --git a/test/years.spec.ts b/test/years.spec.ts
@@ -11,22 +11,6 @@ describe('years.spec.ts', () => {
 		expect(day).to.equal(29);
 	});
 
-	it('1.1 correctly subtract from a leap year when the current month has more days than the resulting one', () => {
-		sDt = Simply.from.date(new Date('2020-02-29T03:24:00'));
-		const res = Simply.subtract(1).years.from(sDt);
-
-		expect(res.year).to.equal(2019);
-		expect(res.day).to.equal(28);
-	});
-
-	it('1.2 correctly subtract from a leap year when the resulting year is also a leap year', () => {
-		sDt = Simply.from.date(new Date('2024-02-29T03:24:00'));
-		const res = Simply.subtract(4).years.from(sDt);
-
-		expect(res.year).to.equal(2020);
-		expect(res.day).to.equal(29);
-	});
-
 	it('1.3 correctly add years', () => {
 		sDt = Simply.from.date(new Date('2017-03-29T03:24:00'));
 		const { year, day, month } = Simply.add(10).years.to(sDt);
@@ -35,19 +19,39 @@ describe('years.spec.ts', () => {
 		expect(day).to.equal(29);
 	});
 
-	it('1.4 correctly add to a leap year', () => {
-		sDt = Simply.from.date(new Date('2020-02-29T03:24:00'));
-		const { year, day, month } = Simply.add(1).years.to(sDt);
-		expect(year).to.equal(2021);
-		expect(month).to.equal(2);
-		expect(day).to.equal(28);
-	});
+	describe('Edge cases', () => {
+		it('1.1 correctly subtract from a leap year when the current month has more days than the resulting one', () => {
+			sDt = Simply.from.date(new Date('2020-02-29T03:24:00'));
+			const { year, month, day } = Simply.subtract(1).years.from(sDt);
+
+			expect(year).to.equal(2019);
+			expect(month).to.equal(3);
+			expect(day).to.equal(1);
+		});
+
+		it('1.2 correctly subtract from a leap year when the resulting year is also a leap year', () => {
+			sDt = Simply.from.date(new Date('2024-02-29T03:24:00'));
+			const res = Simply.subtract(4).years.from(sDt);
+
+			expect(res.year).to.equal(2020);
+			expect(res.day).to.equal(29);
+		});
+
+		it('1.4 correctly add to a leap year', () => {
+			sDt = Simply.from.date(new Date('2020-02-29T03:24:00'));
+			const { year, day, month } = Simply.add(1).years.to(sDt);
+			expect(year).to.equal(2021);
+			expect(month).to.equal(3);
+			expect(day).to.equal(1);
+		});
+
+		it('1.5 correctly add to a leap year when the result is also a leap year', () => {
+			sDt = Simply.from.date(new Date('2020-02-29T03:24:00'));
+			const { year, day, month } = Simply.add(4).years.to(sDt);
+			expect(year).to.equal(2024);
+			expect(month).to.equal(2);
+			expect(day).to.equal(29);
+		});
 
-	it('1.5 correctly add to a leap year when the result is also a leap year', () => {
-		sDt = Simply.from.date(new Date('2020-02-29T03:24:00'));
-		const { year, day, month } = Simply.add(4).years.to(sDt);
-		expect(year).to.equal(2024);
-		expect(month).to.equal(2);
-		expect(day).to.equal(29);
 	});
 });