Add tests for new GregorianDay methods & fix bugs

Sources/HandySwift/Types/GregorianDay.swift

@@ -89,7 +89,7 @@ public struct GregorianDay {
    /// ```
    public func advanced(byMonths months: Int) -> Self {
       let (overflowingYears, newMonth) = (self.month + months - 1).quotientAndRemainder(dividingBy: 12)
-      return self.with { $0.year += overflowingYears; $0.month = newMonth }
+      return self.with { $0.year += overflowingYears; $0.month = newMonth + 1 }
    }
 
    /// Reverses the date by the specified number of months.
@@ -133,7 +133,7 @@ public struct GregorianDay {
    /// let yesterday = GregorianDay.today.reversed(byYears: 1)
    /// ```
    public func reversed(byYears years: Int) -> Self {
-      self.advanced(byMonths: -years)
+      self.advanced(byYears: -years)
    }
 
    /// Returns the start of the day represented by the date.

Tests/HandySwiftTests/Structs/GregorianDayTests.swift

@@ -0,0 +1,24 @@
+import Foundation
+
+@testable import HandySwift
+import XCTest
+
+final class GregorianDayTests: XCTestCase {
+   func testAdvancedByMonths() {
+      let day = GregorianDay(year: 2024, month: 03, day: 26)
+      let advancedByAMonth = day.advanced(byMonths: 1)
+
+      XCTAssertEqual(advancedByAMonth.year, 2024)
+      XCTAssertEqual(advancedByAMonth.month, 04)
+      XCTAssertEqual(advancedByAMonth.day, 26)
+   }
+
+   func testReversedByYears() {
+      let day = GregorianDay(year: 2024, month: 03, day: 26)
+      let reversedByTwoYears = day.reversed(byYears: 2)
+
+      XCTAssertEqual(reversedByTwoYears.year, 2022)
+      XCTAssertEqual(reversedByTwoYears.month, 03)
+      XCTAssertEqual(reversedByTwoYears.day, 26)
+   }
+}