src/buddhist/lib/format.ts

import { format as dfFormat } from "date-fns";
import type { DateLibOptions } from "../../classes/DateLib.js";

/** Format override adding +543 to year tokens for Buddhist Era (BE). */
export function format(
  date: Date,
  formatStr: string,
  options?: DateLibOptions,
): string {
  const beYear = date.getFullYear() + 543;

  switch (formatStr) {
    case "LLLL y":
    case "LLLL yyyy":
      return `${dfFormat(date, "LLLL", options)} ${beYear}`;
    case "LLLL":
      return dfFormat(date, "LLLL", options);
    case "yyyy":
      return String(beYear).padStart(4, "0");
    case "y":
      return String(beYear);
    case "yyyy-MM":
      return `${beYear}-${dfFormat(date, "MM", options)}`;
    case "yyyy-MM-dd":
      return `${beYear}-${dfFormat(date, "MM", options)}-${dfFormat(date, "dd", options)}`;
    case "PPP":
    case "PPPP": {
      const raw = dfFormat(date, formatStr, options);
      return raw.replace(/(.*)(\d{4})(?!.*\d)/, (_m, pre) => `${pre}${beYear}`);
    }
    default:
      return dfFormat(date, formatStr, options);
  }
}