fix: use local timezone instead of UTC for date calculations to prevent early day rollover

Author: rogeriopradoj

src/js/main.js

@@ -1,11 +1,8 @@
 import dayjs from 'dayjs';
-import utc from 'dayjs/plugin/utc.js';
 import * as bootstrap from 'bootstrap';
 import { daysUntilNextBirthday } from './modules/days.js';
 import { ver } from './modules/version.js';
 
-dayjs.extend(utc);
-
 // Initialize when DOM is loaded
 document.addEventListener('DOMContentLoaded', () => {
     // Bootstrap modals are initialized automatically via data-attributes
@@ -68,8 +65,8 @@ document.addEventListener('DOMContentLoaded', () => {
         const dateStart = document.getElementById('dateStart').value;
         const dateEnd = document.getElementById('dateEnd').value;
 
-        const startDate = dayjs.utc(dateStart);
-        const endDate = dayjs.utc(dateEnd);
+        const startDate = dayjs(dateStart);
+        const endDate = dayjs(dateEnd);
 
         let c = ver(startDate, endDate);
 

src/js/modules/days.js

@@ -1,10 +1,7 @@
 import dayjs from 'dayjs';
-import utc from 'dayjs/plugin/utc.js';
-
-dayjs.extend(utc);
 
 /**
- * Returns number of days until next birthday from now (UTC)
+ * Returns number of days until next birthday from now (local timezone)
  * @param {Date|string|number} birthday
  * @returns {number}
  */
@@ -13,17 +10,17 @@ export function daysUntilNextBirthday(birthday) {
 }
 
 /**
- * Returns number of days until next birthday from a given 'now' date (UTC)
+ * Returns number of days until next birthday from a given 'now' date (local timezone)
  * @param {Date|string|number} birthday
  * @param {Date|string|number} nowMoment
  * @returns {number}
  */
 export function daysUntilNextBirthdayGivenNow(birthday, nowMoment) {
-  const b = dayjs.utc(birthday).startOf('day');
-  const now = dayjs.utc(nowMoment).startOf('day');
+  const b = dayjs(birthday).startOf('day');
+  const now = dayjs(nowMoment).startOf('day');
 
   const thisYear = now.year();
-  const birthdayThisYear = dayjs.utc(`${thisYear}-${b.format('MM')}-${b.format('DD')}`).startOf('day');
+  const birthdayThisYear = dayjs(`${thisYear}-${b.format('MM')}-${b.format('DD')}`).startOf('day');
   const birthdayNextYear = birthdayThisYear.add(1, 'year').startOf('day');
 
   const hadBirthdayThisYear = birthdayThisYear.isBefore(now) || birthdayThisYear.isSame(now);

src/js/modules/version.js

@@ -1,11 +1,8 @@
 import dayjs from 'dayjs';
-import utc from 'dayjs/plugin/utc.js';
-
-dayjs.extend(utc);
 
 export function ver(startDate, endDate) {
-    const start = dayjs.utc(startDate);
-    const end = dayjs.utc(endDate);
+    const start = dayjs(startDate);
+    const end = dayjs(endDate);
 
     const years = end.diff(start, 'year');
     const months = end.diff(start, 'month') % 12;