add explicit type for string-based datetimes, replacing third-party datetime type with internal DateTime type

src/core/datetime.ts

@@ -1,5 +1,27 @@
 import type { TypeAndName, TypeAndDisplayName } from '@/core/base.ts';
 
+export interface DateTime {
+    getUnixTime(): number;
+    getLocalizedCalendarYear(): number;
+    getGregorianCalendarYear(): number;
+    getGregorianCalendarQuarter(): number;
+    getLocalizedCalendarQuarter(): number;
+    getGregorianCalendarMonth(): number;
+    getGregorianCalendarMonthName(): string;
+    getLocalizedCalendarMonth(): number;
+    getGregorianCalendarDay(): number;
+    getLocalizedCalendarDay(): number;
+    getGregorianCalendarYearDashMonthDashDay(): TextualYearMonthDay;
+    getGregorianCalendarYearDashMonth(): TextualYearMonth;
+    getWeekDay(): WeekDay;
+    toGregorianCalendarYearMonthDay(): YearMonthDay;
+    toGregorianCalendarYear0BasedMonth(): Year0BasedMonth;
+    format(format: string): string;
+}
+
+export type TextualYearMonth = `${number}-${number}`;
+export type TextualYearMonthDay = `${number}-${number}-${number}`;
+
 export interface YearQuarter {
     readonly year: number;
     readonly quarter: number;

src/core/fiscalyear.ts

@@ -1,4 +1,4 @@
-import type { UnixTimeRange } from './datetime.ts';
+import type { TextualYearMonth, UnixTimeRange } from './datetime.ts';
 
 export class FiscalYearStart {
     public static readonly JanuaryFirstDay = new FiscalYearStart(1, 1);
@@ -75,8 +75,8 @@ export class FiscalYearStart {
         return FiscalYearStart.of(month, day);
     }
 
-    public toMonthDashDayString(): string {
-        return `${this.month.toString().padStart(2, '0')}-${this.day.toString().padStart(2, '0')}`;
+    public toMonthDashDayString(): TextualYearMonth {
+        return `${this.month.toString().padStart(2, '0')}-${this.day.toString().padStart(2, '0')}` as TextualYearMonth;
     }
 
     private static isValidFiscalYearMonthDay(month: number, day: number): boolean {