diff --git a/packages/features/calendars/lib/getCalendarsEvents.test.ts b/packages/features/calendars/lib/getCalendarsEvents.test.ts
index 915425a001..03ebf5ce66 100644
--- a/packages/features/calendars/lib/getCalendarsEvents.test.ts
+++ b/packages/features/calendars/lib/getCalendarsEvents.test.ts
@@ -542,6 +542,162 @@ describe("getCalendarsEventsWithTimezones", () => {
       expect(result).toEqual([[]]);
     });
   });
+
+  describe("Invalid timezone handling", () => {
+    it("should convert GMT-05:00 offset format to Etc/GMT+5", async () => {
+      const availability = [
+        {
+          start: new Date(2010, 11, 2),
+          end: new Date(2010, 11, 3),
+          timeZone: "GMT-05:00",
+        },
+        {
+          start: new Date(2010, 11, 2, 4),
+          end: new Date(2010, 11, 2, 16),
+          timeZone: "America/New_York",
+        },
+      ];
+
+      mockGoogleGetAvailabilityWithTimeZones.mockResolvedValueOnce(availability);
+
+      const selectedCalendar: SelectedCalendar = buildSelectedCalendar({
+        credentialId: 100,
+        externalId: "externalId",
+        integration: "google_calendar",
+        userId: 200,
+        id: "id",
+      });
+      const result = await getCalendarsEventsWithTimezones(
+        [buildRegularCredential(credential)],
+        "2010-12-01",
+        "2010-12-04",
+        [selectedCalendar]
+      );
+
+      expect(result).toEqual([
+        [
+          {
+            start: new Date(2010, 11, 2),
+            end: new Date(2010, 11, 3),
+            timeZone: "Etc/GMT+5",
+          },
+          {
+            start: new Date(2010, 11, 2, 4),
+            end: new Date(2010, 11, 2, 16),
+            timeZone: "America/New_York",
+          },
+        ],
+      ]);
+    });
+
+    it("should convert UTC+08:00 offset format to Etc/GMT-8", async () => {
+      const availability = [
+        {
+          start: new Date(2010, 11, 2),
+          end: new Date(2010, 11, 3),
+          timeZone: "UTC+08:00",
+        },
+      ];
+
+      mockGoogleGetAvailabilityWithTimeZones.mockResolvedValueOnce(availability);
+
+      const selectedCalendar: SelectedCalendar = buildSelectedCalendar({
+        credentialId: 100,
+        externalId: "externalId",
+        integration: "google_calendar",
+        userId: 200,
+        id: "id",
+      });
+      const result = await getCalendarsEventsWithTimezones(
+        [buildRegularCredential(credential)],
+        "2010-12-01",
+        "2010-12-04",
+        [selectedCalendar]
+      );
+
+      expect(result).toEqual([
+        [
+          {
+            start: new Date(2010, 11, 2),
+            end: new Date(2010, 11, 3),
+            timeZone: "Etc/GMT-8",
+          },
+        ],
+      ]);
+    });
+
+    it("should fallback to UTC when timezone is undefined", async () => {
+      const availability = [
+        {
+          start: new Date(2010, 11, 2),
+          end: new Date(2010, 11, 3),
+          timeZone: undefined,
+        },
+      ];
+
+      mockGoogleGetAvailabilityWithTimeZones.mockResolvedValueOnce(availability);
+
+      const selectedCalendar: SelectedCalendar = buildSelectedCalendar({
+        credentialId: 100,
+        externalId: "externalId",
+        integration: "google_calendar",
+        userId: 200,
+        id: "id",
+      });
+      const result = await getCalendarsEventsWithTimezones(
+        [buildRegularCredential(credential)],
+        "2010-12-01",
+        "2010-12-04",
+        [selectedCalendar]
+      );
+
+      expect(result).toEqual([
+        [
+          {
+            start: new Date(2010, 11, 2),
+            end: new Date(2010, 11, 3),
+            timeZone: "UTC",
+          },
+        ],
+      ]);
+    });
+
+    it("should fallback to UTC for completely invalid timezone formats", async () => {
+      const availability = [
+        {
+          start: new Date(2010, 11, 2),
+          end: new Date(2010, 11, 3),
+          timeZone: "InvalidTimezone",
+        },
+      ];
+
+      mockGoogleGetAvailabilityWithTimeZones.mockResolvedValueOnce(availability);
+
+      const selectedCalendar: SelectedCalendar = buildSelectedCalendar({
+        credentialId: 100,
+        externalId: "externalId",
+        integration: "google_calendar",
+        userId: 200,
+        id: "id",
+      });
+      const result = await getCalendarsEventsWithTimezones(
+        [buildRegularCredential(credential)],
+        "2010-12-01",
+        "2010-12-04",
+        [selectedCalendar]
+      );
+
+      expect(result).toEqual([
+        [
+          {
+            start: new Date(2010, 11, 2),
+            end: new Date(2010, 11, 3),
+            timeZone: "UTC",
+          },
+        ],
+      ]);
+    });
+  });
 });
 
 // CalDAV Credential Leak Prevention Tests
diff --git a/packages/features/calendars/lib/getCalendarsEvents.ts b/packages/features/calendars/lib/getCalendarsEvents.ts
index 76d023ddfe..5277a74d28 100644
--- a/packages/features/calendars/lib/getCalendarsEvents.ts
+++ b/packages/features/calendars/lib/getCalendarsEvents.ts
@@ -8,9 +8,12 @@ import { performance } from "@calcom/lib/server/perfObserver";
 import type { CalendarFetchMode, EventBusyDate, SelectedCalendar } from "@calcom/types/Calendar";
 import type { CredentialForCalendarService } from "@calcom/types/Credential";
 
+import { normalizeTimezone } from "./timezone-conversion";
+
 const log = logger.getSubLogger({ prefix: ["getCalendarsEvents"] });
 
 const CALENDSO_ENCRYPTION_KEY = process.env.CALENDSO_ENCRYPTION_KEY || "";
+
 // only for Google Calendar for now
 export const getCalendarsEventsWithTimezones = async (
   withCredentials: CredentialForCalendarService[],
@@ -77,7 +80,7 @@ export const getCalendarsEventsWithTimezones = async (
 
     return eventBusyDates.map((event) => ({
       ...event,
-      timeZone: event.timeZone || "UTC",
+      timeZone: normalizeTimezone(event.timeZone),
     }));
   });
   const awaitedResults = await Promise.all(results);
diff --git a/packages/features/calendars/lib/timezone-conversion.test.ts b/packages/features/calendars/lib/timezone-conversion.test.ts
new file mode 100644
index 0000000000..a04def7d8e
--- /dev/null
+++ b/packages/features/calendars/lib/timezone-conversion.test.ts
@@ -0,0 +1,108 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { convertOffsetToIanaTimezone, normalizeTimezone } from "./timezone-conversion";
+
+vi.mock("@calcom/lib/logger", () => ({
+  default: {
+    getSubLogger: () => ({
+      info: vi.fn(),
+      warn: vi.fn(),
+    }),
+  },
+}));
+
+describe("timezone-conversion", () => {
+  describe("Intl.DateTimeFormat behavior with offset formats", () => {
+    it("throws RangeError for GMT-05:00 offset format", () => {
+      expect(() => Intl.DateTimeFormat(undefined, { timeZone: "GMT-05:00" })).toThrow(RangeError);
+    });
+
+    it("throws RangeError for UTC+08:00 offset format", () => {
+      expect(() => Intl.DateTimeFormat(undefined, { timeZone: "UTC+08:00" })).toThrow(RangeError);
+    });
+
+    it("throws RangeError for GMT+5 offset format without minutes", () => {
+      expect(() => Intl.DateTimeFormat(undefined, { timeZone: "GMT+5" })).toThrow(RangeError);
+    });
+
+    it("accepts valid IANA timezone America/New_York", () => {
+      expect(() => Intl.DateTimeFormat(undefined, { timeZone: "America/New_York" })).not.toThrow();
+    });
+
+    it("accepts Etc/GMT+5 as valid IANA timezone", () => {
+      expect(() => Intl.DateTimeFormat(undefined, { timeZone: "Etc/GMT+5" })).not.toThrow();
+    });
+
+    it("accepts UTC as valid timezone", () => {
+      expect(() => Intl.DateTimeFormat(undefined, { timeZone: "UTC" })).not.toThrow();
+    });
+  });
+
+  describe("convertOffsetToIanaTimezone", () => {
+    it("converts GMT-05:00 to Etc/GMT+5 (inverted sign)", () => {
+      expect(convertOffsetToIanaTimezone("GMT-05:00")).toBe("Etc/GMT+5");
+    });
+
+    it("converts UTC+08:00 to Etc/GMT-8 (inverted sign)", () => {
+      expect(convertOffsetToIanaTimezone("UTC+08:00")).toBe("Etc/GMT-8");
+    });
+
+    it("converts GMT+0 to Etc/GMT", () => {
+      expect(convertOffsetToIanaTimezone("GMT+0")).toBe("Etc/GMT");
+    });
+
+    it("converts GMT-00:00 to Etc/GMT", () => {
+      expect(convertOffsetToIanaTimezone("GMT-00:00")).toBe("Etc/GMT");
+    });
+
+    it("handles case insensitivity (gmt-05:00)", () => {
+      expect(convertOffsetToIanaTimezone("gmt-05:00")).toBe("Etc/GMT+5");
+    });
+
+    it("handles format without minutes (GMT+5)", () => {
+      expect(convertOffsetToIanaTimezone("GMT+5")).toBe("Etc/GMT-5");
+    });
+
+    it("returns null for non-zero minutes (cannot be represented in Etc/GMT)", () => {
+      expect(convertOffsetToIanaTimezone("GMT+05:30")).toBeNull();
+    });
+
+    it("returns null for hours > 14", () => {
+      expect(convertOffsetToIanaTimezone("GMT+15:00")).toBeNull();
+    });
+
+    it("returns null for invalid format", () => {
+      expect(convertOffsetToIanaTimezone("America/New_York")).toBeNull();
+    });
+
+    it("returns null for completely invalid string", () => {
+      expect(convertOffsetToIanaTimezone("InvalidTimezone")).toBeNull();
+    });
+  });
+
+  describe("normalizeTimezone", () => {
+    it("returns UTC for undefined timezone", () => {
+      expect(normalizeTimezone(undefined)).toBe("UTC");
+    });
+
+    it("returns valid IANA timezone as-is", () => {
+      expect(normalizeTimezone("America/New_York")).toBe("America/New_York");
+    });
+
+    it("converts GMT-05:00 to Etc/GMT+5", () => {
+      expect(normalizeTimezone("GMT-05:00")).toBe("Etc/GMT+5");
+    });
+
+    it("converts UTC+08:00 to Etc/GMT-8", () => {
+      expect(normalizeTimezone("UTC+08:00")).toBe("Etc/GMT-8");
+    });
+
+    it("falls back to UTC for completely invalid timezone", () => {
+      expect(normalizeTimezone("InvalidTimezone")).toBe("UTC");
+    });
+
+    it("falls back to UTC for non-zero minutes offset", () => {
+      expect(normalizeTimezone("GMT+05:30")).toBe("UTC");
+    });
+  });
+});
diff --git a/packages/features/calendars/lib/timezone-conversion.ts b/packages/features/calendars/lib/timezone-conversion.ts
new file mode 100644
index 0000000000..34ed422bf8
--- /dev/null
+++ b/packages/features/calendars/lib/timezone-conversion.ts
@@ -0,0 +1,43 @@
+import logger from "@calcom/lib/logger";
+
+const log = logger.getSubLogger({ prefix: ["timezoneConversion"] });
+
+/**
+ * Converts offset-based timezone formats (GMT±HH:MM, UTC±HH:MM) to IANA Etc/GMT timezones.
+ * Etc/GMT uses inverted signs: Etc/GMT+5 = UTC-5, Etc/GMT-8 = UTC+8.
+ */
+export function convertOffsetToIanaTimezone(timezone: string): string | null {
+  const match = timezone.match(/^(?:GMT|UTC)([+-])(\d{1,2})(?::(\d{2}))?$/i);
+  if (!match) return null;
+
+  const [, sign, hoursStr, minutesStr] = match;
+  const hours = parseInt(hoursStr, 10);
+  const minutes = minutesStr ? parseInt(minutesStr, 10) : 0;
+
+  // Etc/GMT only supports whole hours up to 14
+  if (minutes !== 0 || hours > 14) return null;
+
+  if (hours === 0) return "Etc/GMT";
+  return `Etc/GMT${sign === "+" ? "-" : "+"}${hours}`;
+}
+
+/**
+ * Validates and normalizes a timezone string to a valid IANA timezone.
+ * Converts offset formats to Etc/GMT, falls back to UTC if invalid.
+ */
+export function normalizeTimezone(timezone: string | undefined): string {
+  if (!timezone) return "UTC";
+
+  try {
+    Intl.DateTimeFormat(undefined, { timeZone: timezone });
+    return timezone;
+  } catch {
+    const converted = convertOffsetToIanaTimezone(timezone);
+    if (converted) {
+      log.info(`Converted offset timezone to IANA format`, { original: timezone, converted });
+      return converted;
+    }
+    log.warn(`Invalid timezone format, falling back to UTC`, { invalidTimezone: timezone });
+    return "UTC";
+  }
+}