support calendar display type (Gregorian and Buddhist)

src/core/fiscalyear.ts

@@ -1,4 +1,4 @@
-import type { TextualYearMonth, UnixTimeRange } from './datetime.ts';
+import type { TextualYearMonth, MonthDay, UnixTimeRange } from './datetime.ts';
 
 export class FiscalYearStart {
     public static readonly JanuaryFirstDay = new FiscalYearStart(1, 1);
@@ -79,6 +79,13 @@ export class FiscalYearStart {
         return `${this.month.toString().padStart(2, '0')}-${this.day.toString().padStart(2, '0')}` as TextualYearMonth;
     }
 
+    public toMonthDay(): MonthDay {
+        return {
+            month: this.month,
+            day: this.day
+        };
+    }
+
     private static isValidFiscalYearMonthDay(month: number, day: number): boolean {
         return 1 <= month && month <= 12 && 1 <= day && day <= FiscalYearStart.MONTH_MAX_DAYS[month - 1];
     }