Java 獲取某日是當月幾號的標準 API：Calendar.DAY_OF_MONTH

標準代碼

int dayOfMonth = Calendar.getInstance().get(Calendar.DAY_OF_MONTH);
// 返回 1-31，對應當月具體日期

完整示例

Calendar cal = Calendar.getInstance();
int year = cal.get(Calendar.YEAR);
int month = cal.get(Calendar.MONTH) + 1; // 0-based → 1-12
int day = cal.get(Calendar.DAY_OF_MONTH); // 1-31
int hour = cal.get(Calendar.HOUR_OF_DAY);
int minute = cal.get(Calendar.MINUTE);

System.out.printf("今天是 %d 年 %d 月 %d 日 %02d:%02d%n", year, month, day, hour, minute);

Java 8+ 現代 API (推薦)

LocalDate today = LocalDate.now();
int dayOfMonth = today.getDayOfMonth(); // 1-31
int month = today.getMonthValue();      // 1-12
int year = today.getYear();

// 格式化輸出
String formatted = today.format(DateTimeFormatter.ofPattern("yyyy年MM月dd日"));

Android 兼容 Java 8 (API 26+ / core library desugaring)

LocalDate today = LocalDate.now();
int day = today.getDayOfMonth();
// 需在 build.gradle 啟用 coreLibraryDesugaringEnabled true

總結

Calendar.DAY_OF_MONTH (或 LocalDate.getDayOfMonth()) 直接獲取當月具體日期 (1-31)。棄用手動計算，使用標準 API 避免時區、閏年等邊界錯誤。