Calculate week number

README.md

@@ -46,6 +46,7 @@ void main() {
 -  `isAfterOrEqual`: Checks if the current DateTime is after or equal to the given DateTime
 - `isBeforeOrEqual`:  Checks if the current DateTime is before or equal to the given DateTime
 - `isBetween`:  Checks if the current DateTime is between the given dates
+- `calendarWeek`: calculates the calendar week number of the current DateTime object.
 
 Example:
 

lib/extensions/date_time.extension.dart

@@ -63,4 +63,43 @@ extension DateTimeExtension on DateTime {
     final isBefore = isBeforeOrEqual(toDateTime);
     return isAfter && isBefore;
   }
+
+  ///
+  /// Returns the calendar week of the year for this date.
+  ///
+  int calendarWeek() {
+    // Add 3 to always compare with January 4th, which is always in week 1
+    // Add 7 to index weeks starting with 1 instead of 0
+    final weekOfYear = ((ordinalDate - weekday + 10) ~/ 7);
+
+    // If the week number equals zero, it means that the given date belongs to the preceding (week-based) year.
+    if (weekOfYear == 0) {
+      // The 28th of December is always in the last week of the year
+      return DateTime(year - 1, 12, 28).calendarWeek();
+    }
+
+    // If the week number equals 53, one must check that the date is not actually in week 1 of the following year
+    if (weekOfYear == 53 &&
+        DateTime(year, 1, 1).weekday != DateTime.thursday &&
+        DateTime(year, 12, 31).weekday != DateTime.thursday) {
+      return 1;
+    }
+
+    return weekOfYear;
+  }
+
+  /// The ordinal date, the number of days since December 31st the previous year.
+  ///
+  /// January 1st has the ordinal date 1
+  ///
+  /// December 31st has the ordinal date 365, or 366 in leap years
+  int get ordinalDate {
+    const offsets = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334];
+    return offsets[month - 1] + day + (isLeapYear && month > 2 ? 1 : 0);
+  }
+
+  /// True if this date is on a leap year.
+  bool get isLeapYear {
+    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
+  }
 }