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src/common/utils/timezone.util.ts

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+function extractDateParts(date: Date, timeZone: string) {
+  const formatter = new Intl.DateTimeFormat("en-CA", {
+    timeZone,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+  });
+
+  const parts = formatter.formatToParts(date);
+  const year = Number(parts.find((part) => part.type === "year")?.value);
+  const month = Number(parts.find((part) => part.type === "month")?.value);
+  const day = Number(parts.find((part) => part.type === "day")?.value);
+
+  return { year, month, day };
+}
+
+export function getDateStringInTimeZone(
+  date: Date,
+  timeZone: string,
+): string {
+  const { year, month, day } = extractDateParts(date, timeZone);
+  return `${year}-${String(month).padStart(2, "0")}-${String(day).padStart(2, "0")}`;
+}
+
+export function getShiftedDateStringInTimeZone(
+  daysOffset: number,
+  timeZone: string,
+  baseDate: Date = new Date(),
+): string {
+  const { year, month, day } = extractDateParts(baseDate, timeZone);
+  const shifted = new Date(Date.UTC(year, month - 1, day));
+  shifted.setUTCDate(shifted.getUTCDate() + daysOffset);
+  return shifted.toISOString().split("T")[0];
+}
+
+function getTimeZoneOffsetMs(date: Date, timeZone: string): number {
+  const formatter = new Intl.DateTimeFormat("en-US", {
+    timeZone,
+    timeZoneName: "shortOffset",
+  });
+
+  const offsetLabel =
+    formatter.formatToParts(date).find((part) => part.type === "timeZoneName")
+      ?.value || "GMT+0";
+
+  const match = offsetLabel.match(/GMT([+-])(\d{1,2})(?::?(\d{2}))?/);
+  if (!match) return 0;
+
+  const sign = match[1] === "-" ? -1 : 1;
+  const hours = Number(match[2] || "0");
+  const minutes = Number(match[3] || "0");
+
+  return sign * (hours * 60 + minutes) * 60 * 1000;
+}
+
+export function getDayBoundsForTimeZone(
+  dateString: string,
+  timeZone: string,
+): { startMs: number; endMs: number } {
+  const [year, month, day] = dateString.split("-").map(Number);
+  const startGuess = new Date(Date.UTC(year, month - 1, day, 0, 0, 0));
+  const nextDayGuess = new Date(Date.UTC(year, month - 1, day + 1, 0, 0, 0));
+
+  const startOffsetMs = getTimeZoneOffsetMs(startGuess, timeZone);
+  const nextDayOffsetMs = getTimeZoneOffsetMs(nextDayGuess, timeZone);
+
+  const startMs = Date.UTC(year, month - 1, day, 0, 0, 0) - startOffsetMs;
+  const nextDayStartMs =
+    Date.UTC(year, month - 1, day + 1, 0, 0, 0) - nextDayOffsetMs;
+
+  return {
+    startMs,
+    endMs: nextDayStartMs - 1,
+  };
+}
+
+export function getDateOnlyValueForTimeZone(
+  timeZone: string,
+  date: Date = new Date(),
+): Date {
+  return new Date(`${getDateStringInTimeZone(date, timeZone)}T00:00:00.000Z`);
+}