YearWeek adjustments

src/dateRange.ts

@@ -183,9 +183,35 @@ export class YearMonth extends DateRange implements IYearMonthDateRange {
     }
 }
 
-export const getWeekNumber = (date: Date) => {
-    const onejan = new Date(date.getFullYear(), 0, 1);
-    return Math.ceil(((date.getTime() - onejan.getTime()) / 86400000 + onejan.getDay() + 1) / 7);
+const getStartOfWeek = (date: Date) => {
+    const startOfWeek = new Date(date);
+    const diffDays = startOfWeek.getDay();
+    startOfWeek.setDate(startOfWeek.getDate() - diffDays);
+    startOfWeek.setHours(0, 0, 0, 0);
+    return startOfWeek;
+};
+
+const areDatesInSameWeek = (date1: Date, date2: Date) => {
+    const startOfWeek1 = getStartOfWeek(date1);
+    const startOfWeek2 = getStartOfWeek(date2);
+    return startOfWeek1.getTime() === startOfWeek2.getTime();
+};
+
+export const getWeekNumber = (date: Date): number => {
+    const jan1st = new Date(date.getFullYear(), 0, 1);
+    const daysFromJan1UntilDate = Math.floor((date.getTime() - jan1st.getTime()) / (24 * 60 * 60 * 1000));
+    const offsetFromJan1ToSundaySameWeek = -(jan1st.getDay() % 7) % 7;
+    const weeksRaw = (daysFromJan1UntilDate + offsetFromJan1ToSundaySameWeek) / 7;
+
+    const weekOfDate =
+        (weeksRaw >= 0 ? Math.floor(weeksRaw) : Math.ceil(weeksRaw)) + (daysFromJan1UntilDate !== 365 ? 1 : 0);
+
+    const lastDayOfPreviousYear = new Date(date.getFullYear() - 1, 11, 31);
+
+    if (weekOfDate === 1 && areDatesInSameWeek(lastDayOfPreviousYear, date))
+        return getWeekNumber(lastDayOfPreviousYear);
+
+    return weekOfDate;
 };
 
 export const getWeekOfYear = (): number => getWeekNumber(new Date());

src/dateUtils.ts

@@ -10,14 +10,14 @@ const formatWeekDaysRange = (start: Date, end: Date) =>
 
 export const getWeekDaysRange = (week: number, year?: number) => {
     year = year ?? new Date().getFullYear();
-    const firstDayOfYear = new Date(year, 0, 1);
 
-    let pivotDay = 1 + (week - 1) * 7 - firstDayOfYear.getDay();
+    const jan1st = new Date(year, 0, 1);
+    const offsetToFirstSunday = (7 - jan1st.getDay()) % 7;
+    const weekStart = new Date(jan1st);
+    weekStart.setDate(jan1st.getDate() + offsetToFirstSunday + (week - 1) * 7);
 
-    if (firstDayOfYear.getDay() > 4) pivotDay += 7;
-
-    const weekStart = new Date(year, 0, pivotDay);
-    const weekEnd = new Date(year, 0, pivotDay + 6);
+    const weekEnd = new Date(weekStart);
+    weekEnd.setDate(weekStart.getDate() + 6);
 
     return formatWeekDaysRange(weekStart, weekEnd);
 };