From 452766a2b681511f7ce1383b78057c6ca68d3011 Mon Sep 17 00:00:00 2001
From: Chayan Mistry <chayanforyou@yahoo.com>
Date: Tue, 5 Sep 2023 15:36:41 +0600
Subject: [PATCH] Adjust months and years based on the days overflow

---
 README.md               | 29 ++++++++++++++++++++++++++---
 example/lib/main.dart   | 25 ++++++++++++++++++++++++-
 lib/age_calculator.dart | 32 ++++++++++++++++++++++++++------
 3 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/README.md b/README.md
index 6e60e53..13b2925 100644
--- a/README.md
+++ b/README.md
@@ -53,10 +53,33 @@ void main() {
   // You next birthday will be in Years: 4, Months: 4, Days: 0
 
   // Add time to any date
-  DateTime date = AgeCalculator.add(
-      date: DateTime(2021, 1, 2),
-      duration: DateDuration(years: 5, months: 2, days: 1));
+  DateTime date;
+
+  print('\nAdd Example:');
+  date = AgeCalculator.add(
+          date: DateTime(2021, 1, 2),
+          duration: DateDuration(years: 5, months: 2, days: 1));
   print(date);
   // 2026-03-03 00:00:00.000
+
+  date = AgeCalculator.add(
+          date: DateTime(2023, 2, 28),
+          duration: DateDuration(months: 6));
+  print(date);
+  // 2024-02-29 00:00:00.000
+
+  // Subtract time to any date
+  print('\nSubtract Example:');
+  date = AgeCalculator.subtract(
+          date: DateTime(2021, 1, 2),
+          duration: DateDuration(years: 5, months: 2, days: 1));
+  print(date);
+  // 2015-11-01 00:00:00.000
+
+  date = AgeCalculator.subtract(
+          date: DateTime(2023, 8, 31),
+          duration: DateDuration(months: 6));
+  print(date);
+  // 2023-02-28 00:00:00.000
 }
 ```
diff --git a/example/lib/main.dart b/example/lib/main.dart
index 46ec736..62ff036 100644
--- a/example/lib/main.dart
+++ b/example/lib/main.dart
@@ -33,9 +33,32 @@ void main() {
   // You next birthday will be in Years: 4, Months: 4, Days: 0
 
   // Add time to any date
-  DateTime date = AgeCalculator.add(
+  DateTime date;
+
+  print('\nAdd Example:');
+  date = AgeCalculator.add(
       date: DateTime(2021, 1, 2),
       duration: DateDuration(years: 5, months: 2, days: 1));
   print(date);
   // 2026-03-03 00:00:00.000
+
+  date = AgeCalculator.add(
+      date: DateTime(2023, 2, 28),
+      duration: DateDuration(months: 6));
+  print(date);
+  // 2024-02-29 00:00:00.000
+
+  // Subtract time to any date
+  print('\nSubtract Example:');
+  date = AgeCalculator.subtract(
+      date: DateTime(2021, 1, 2),
+      duration: DateDuration(years: 5, months: 2, days: 1));
+  print(date);
+  // 2015-11-01 00:00:00.000
+
+  date = AgeCalculator.subtract(
+      date: DateTime(2023, 8, 31),
+      duration: DateDuration(months: 6));
+  print(date);
+  // 2023-02-28 00:00:00.000
 }
diff --git a/lib/age_calculator.dart b/lib/age_calculator.dart
index 8fd12c7..d019f7c 100644
--- a/lib/age_calculator.dart
+++ b/lib/age_calculator.dart
@@ -1,5 +1,7 @@
 library age_calculator;
 
+import 'dart:math';
+
 class DateDuration {
   int days;
   int months;
@@ -95,13 +97,19 @@ class AgeCalculator {
   /// add method
   static DateTime add(
       {required DateTime date, required DateDuration duration}) {
-    int years = date.year + duration.years;
-    years += ((date.month + duration.months) ~/ DateTime.monthsPerYear);
-    int months = ((date.month + duration.months) % DateTime.monthsPerYear);
-
-    int days = date.day + duration.days - 1;
+    var newDateTime = date.add(Duration(days: duration.days));
+    newDateTime = _addMonths(newDateTime, duration.months);
+    newDateTime = _addMonths(newDateTime, duration.years * 12);
+    return newDateTime;
+  }
 
-    return DateTime(years, months, 1).add(Duration(days: days));
+  /// subtract methods
+  static DateTime subtract(
+      {required DateTime date, required DateDuration duration}) {
+    var newDateTime = date.subtract(Duration(days: duration.days));
+    newDateTime = _addMonths(newDateTime, -duration.months);
+    newDateTime = _addMonths(newDateTime, -duration.years * 12);
+    return newDateTime;
   }
 
   static DateDuration age(DateTime birthdate, {DateTime? today}) {
@@ -117,4 +125,16 @@ class AgeCalculator {
         : tempDate;
     return dateDifference(fromDate: endDate, toDate: nextBirthdayDate);
   }
+
+  static DateTime _addMonths(DateTime dateTime, int months) {
+    final modMonths = months % 12;
+    var newYear = dateTime.year + ((months - modMonths) ~/ 12);
+    var newMonth = dateTime.month + modMonths;
+    if (newMonth > 12) {
+      newYear++;
+      newMonth -= 12;
+    }
+    final newDay = min(dateTime.day, daysInMonth(newYear, newMonth));
+    return DateTime(newYear, newMonth, newDay);
+  }
 }