diff --git a/common/src/mayan.test.ts b/common/src/mayan.test.ts
index d59aee3..9bcc8e3 100644
--- a/common/src/mayan.test.ts
+++ b/common/src/mayan.test.ts
@@ -2,7 +2,7 @@ import {
     formatHaab,
     formatLordOfNight,
     formatTzolkin, HaabMonth, haabMonthDays,
-    jdnHaab, jdnHaabExt,
+    jdnHaab, jdnHaabExt, haabExtJDN,
     jdnLordOfNight,
     jdnTzolkin,
     TzolkinName,
@@ -207,6 +207,38 @@ describe('jdnHaabExt', () => {
     });
 });
 
+describe('haabExtJDN', () => {
+    it('handles creation correctly', () => {
+        expect(haabExtJDN({year: -1, month: 19, day: 4})).toBe(583934); // end of the year before creation
+        expect(haabExtJDN({year: 0, month: 1, day: 0})).toBe(583935); // start of the year of creation
+        expect(haabExtJDN({year: 0, month: 18, day: 8})).toBe(584283); // creation
+        expect(haabExtJDN({year: 0, month: 19, day: 4})).toBe(584299); // end of the year of creation
+        expect(haabExtJDN({year: 1, month: 1, day: 0})).toBe(584300); // first 0 Pop after creation
+    });
+
+    it('handles 5141 cycles since creation correctly', () => {
+        expect(haabExtJDN({year: 5141, month: 18, day: 8})).toBe(2460748); // exactly 5141 years since creation
+        expect(haabExtJDN({year: 5141, month: 19, day: 4})).toBe(2460764); // end of that cycle
+        expect(haabExtJDN({year: 5142, month: 1, day: 0})).toBe(2460765); // start of the next cycle
+    });
+
+    it('converts sample dates from history correctly', () => {
+        expect(haabExtJDN({year: 0, month: 18, day: 8})).toBe(584283); // Mayan creation
+        expect(haabExtJDN({year: 3072, month: 11, day: 11})).toBe(1705426); // Ides of March
+        expect(haabExtJDN({year: 4609, month: 4, day: 16})).toBe(2266296); // Columbus reaches the Americas
+        expect(haabExtJDN({year: 5058, month: 12, day: 11})).toBe(2430336); // a date which will live in infamy
+        expect(haabExtJDN({year: 5086, month: 5, day: 18})).toBe(2440423); // Moon landing
+        expect(haabExtJDN({year: 5136, month: 18, day: 5})).toBe(2458920); // COVID-19 pandemic
+    });
+
+    it('handles negative JDN correctly', () => {
+        expect(haabExtJDN({year: -1601, month: 4, day: 5})).toBe(-365);
+        expect(haabExtJDN({year: -1601, month: 19, day: 4})).toBe(-66);
+        expect(haabExtJDN({year: -1600, month: 1, day: 0})).toBe(-65);
+        expect(haabExtJDN({year: -1600, month: 4, day: 4})).toBe(-1);
+    });
+});
+
 describe('formatLordOfNight', () => {
     it('should format Lord of Night 1', () => {
         expect(formatLordOfNight(1)).toBe('G1');
diff --git a/common/src/mayan.ts b/common/src/mayan.ts
index 9bca945..ce270cd 100644
--- a/common/src/mayan.ts
+++ b/common/src/mayan.ts
@@ -125,6 +125,10 @@ export function jdnHaabExt(jdn: number): HaabExt {
     };
 }
 
+export function haabExtJDN({year, month, day}: HaabExt): number {
+    return 583935 + 365 * year + (month - 1) * 20 + day;
+}
+
 export type LordOfNight = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
 
 export function formatLordOfNight(lordOfNight: LordOfNight): string {