diff --git a/common/src/mayan.test.ts b/common/src/mayan.test.ts
index 6b1fc36..d59aee3 100644
--- a/common/src/mayan.test.ts
+++ b/common/src/mayan.test.ts
@@ -1,7 +1,7 @@
 import {
     formatHaab,
     formatLordOfNight,
-    formatTzolkin,
+    formatTzolkin, HaabMonth, haabMonthDays,
     jdnHaab, jdnHaabExt,
     jdnLordOfNight,
     jdnTzolkin,
@@ -163,6 +163,18 @@ describe('jdnHaab', () => {
     });
 });
 
+describe('haabMonthDays', () => {
+    it('returns 20 days for months 1-18', () => {
+        for (let month = 1; month <= 18; month++) {
+            expect(haabMonthDays(month as HaabMonth)).toBe(20);
+        }
+    });
+
+    it('returns 5 days for month 19 (Wayeb)', () => {
+        expect(haabMonthDays(19)).toBe(5);
+    });
+});
+
 describe('jdnHaabExt', () => {
     it('handles creation correctly', () => {
         expect(jdnHaabExt(583934)).toEqual({year: -1, month: 19, day: 4}); // end of the year before creation
diff --git a/common/src/mayan.ts b/common/src/mayan.ts
index d4d19bd..9bca945 100644
--- a/common/src/mayan.ts
+++ b/common/src/mayan.ts
@@ -108,6 +108,10 @@ export function jdnHaab(jdn: number): Haab {
     };
 }
 
+export function haabMonthDays(month: HaabMonth): number {
+    return month === 19 ? 5 : 20;
+}
+
 export type HaabYear = number;
 
 export type HaabExt = Haab & {