LibJS: Account for differences in month representations (0-11 vs 1-12)

Since DateTime stores months as 1 to 12, while JS accepts months as
0 to 11, we have to account for the difference (by subtracting or
adding 1) where appropriate.

2 files changed, 6 insertions, 6 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp b/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp
index 4f0c958b18..2b59e36945 100644
--- a/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/DatePrototype.cpp
@@ -169,14 +169,14 @@ JS_DEFINE_NATIVE_FUNCTION(DatePrototype::set_full_year)
     }
     auto new_year = new_year_value.as_i32();
 
-    auto new_month_value = arg_or(1, datetime.month());
+    auto new_month_value = arg_or(1, datetime.month() - 1);
     if (vm.exception())
         return {};
     if (!new_month_value.is_finite_number()) {
         this_object->set_is_invalid(true);
         return js_nan();
     }
-    auto new_month = new_month_value.as_i32();
+    auto new_month = new_month_value.as_i32() + 1; // JS Months: 0 - 11, DateTime months: 1-12
 
     auto new_day_value = arg_or(2, datetime.day());
     if (vm.exception())
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.setFullYear.js b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.setFullYear.js
index a9a957b0db..9c5dec7de9 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.setFullYear.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.setFullYear.js
@@ -40,7 +40,7 @@ test("Year and month as arguments", () => {
 
     date.setFullYear(2021, 3);
     expect(date.getFullYear()).toBe(2021);
-    expect(date.getMonth()).toBe(2);
+    expect(date.getMonth()).toBe(3);
     expect(date.getDate()).toBe(1);
     expect(date.getHours()).toBe(0);
     expect(date.getMinutes()).toBe(0);
@@ -53,7 +53,7 @@ test("Year, month, and day as arguments", () => {
 
     date.setFullYear(2021, 3, 16);
     expect(date.getFullYear()).toBe(2021);
-    expect(date.getMonth()).toBe(2);
+    expect(date.getMonth()).toBe(3);
     expect(date.getDate()).toBe(16);
     expect(date.getHours()).toBe(0);
     expect(date.getMinutes()).toBe(0);
@@ -88,7 +88,7 @@ test("NaN or undefined in any arguments", () => {
 
     date.setFullYear(2021, 3, 16);
     expect(date.getFullYear()).toBe(2021);
-    expect(date.getMonth()).toBe(2);
+    expect(date.getMonth()).toBe(3);
     expect(date.getDate()).toBe(16);
     expect(date.getHours()).toBe(0);
     expect(date.getMinutes()).toBe(0);
@@ -103,7 +103,7 @@ test("Make Invalid Date valid again", () => {
 
     date.setFullYear(2021, 3, 16);
     expect(date.getFullYear()).toBe(2021);
-    expect(date.getMonth()).toBe(2);
+    expect(date.getMonth()).toBe(3);
     expect(date.getDate()).toBe(16);
     expect(date.getHours()).toBe(0);
     expect(date.getMinutes()).toBe(0);