diff --git a/lib/validations.normalizeTimezone.test.ts b/lib/validations.normalizeTimezone.test.ts
new file mode 100644
index 000000000..aa878d03f
--- /dev/null
+++ b/lib/validations.normalizeTimezone.test.ts
@@ -0,0 +1,125 @@
+import { describe, it, expect } from 'vitest';
+import { normalizeTimezone } from './validations';
+import { streakParamsSchema } from './validations';
+
+describe('normalizeTimezone', () => {
+  it('maps GMT+N to Etc/GMT-N (inverted sign convention)', () => {
+    expect(normalizeTimezone('GMT+1')).toBe('Etc/GMT-1');
+    expect(normalizeTimezone('GMT+5')).toBe('Etc/GMT-5');
+    expect(normalizeTimezone('GMT+14')).toBe('Etc/GMT-14');
+  });
+
+  it('maps GMT-N to Etc/GMT+N (inverted sign convention)', () => {
+    expect(normalizeTimezone('GMT-1')).toBe('Etc/GMT+1');
+    expect(normalizeTimezone('GMT-5')).toBe('Etc/GMT+5');
+    expect(normalizeTimezone('GMT-12')).toBe('Etc/GMT+12');
+  });
+
+  it('maps UTC+N to Etc/GMT-N', () => {
+    expect(normalizeTimezone('UTC+3')).toBe('Etc/GMT-3');
+    expect(normalizeTimezone('UTC+8')).toBe('Etc/GMT-8');
+  });
+
+  it('maps UTC-N to Etc/GMT+N', () => {
+    expect(normalizeTimezone('UTC-3')).toBe('Etc/GMT+3');
+    expect(normalizeTimezone('UTC-8')).toBe('Etc/GMT+8');
+  });
+
+  it('maps GMT+0 and UTC+0 to UTC', () => {
+    expect(normalizeTimezone('GMT+0')).toBe('UTC');
+    expect(normalizeTimezone('UTC+0')).toBe('UTC');
+  });
+
+  it('maps GMT-0 and UTC-0 to UTC', () => {
+    expect(normalizeTimezone('GMT-0')).toBe('UTC');
+    expect(normalizeTimezone('UTC-0')).toBe('UTC');
+  });
+
+  it('is case-insensitive', () => {
+    expect(normalizeTimezone('gmt+5')).toBe('Etc/GMT-5');
+    expect(normalizeTimezone('utc-3')).toBe('Etc/GMT+3');
+    expect(normalizeTimezone('Gmt+1')).toBe('Etc/GMT-1');
+    expect(normalizeTimezone('Utc-7')).toBe('Etc/GMT+7');
+  });
+
+  it('returns the input unchanged for IANA timezone names', () => {
+    expect(normalizeTimezone('America/New_York')).toBe('America/New_York');
+    expect(normalizeTimezone('Asia/Kolkata')).toBe('Asia/Kolkata');
+    expect(normalizeTimezone('UTC')).toBe('UTC');
+    expect(normalizeTimezone('Etc/GMT+5')).toBe('Etc/GMT+5');
+  });
+
+  it('returns the input unchanged for out-of-range offsets', () => {
+    // UTC+14 is the max positive offset; +15 is out of range
+    expect(normalizeTimezone('GMT+15')).toBe('GMT+15');
+    // UTC-12 is the max negative offset; -13 is out of range
+    expect(normalizeTimezone('GMT-13')).toBe('GMT-13');
+  });
+
+  it('returns the input unchanged for fractional offsets (unsupported)', () => {
+    // Fractional offsets like UTC+5:30 don't match the whole-hour regex
+    expect(normalizeTimezone('UTC+5:30')).toBe('UTC+5:30');
+    expect(normalizeTimezone('GMT+5:45')).toBe('GMT+5:45');
+  });
+
+  it('returns the input unchanged for invalid strings', () => {
+    expect(normalizeTimezone('Invalid/Zone')).toBe('Invalid/Zone');
+    expect(normalizeTimezone('')).toBe('');
+    expect(normalizeTimezone('GMT')).toBe('GMT');
+    expect(normalizeTimezone('UTC')).toBe('UTC');
+  });
+});
+
+describe('streakParamsSchema — tz field with raw GMT/UTC offsets', () => {
+  const baseParams = { user: 'octocat' };
+
+  it('accepts standard IANA timezone names', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'America/New_York' });
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts Etc/GMT±N format', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'Etc/GMT+5' });
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts raw GMT+N offsets', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'GMT+1' });
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts raw GMT-N offsets', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'GMT-5' });
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts raw UTC+N offsets', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'UTC+8' });
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts raw UTC-N offsets', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'UTC-3' });
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts GMT+0 as valid', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'GMT+0' });
+    expect(result.success).toBe(true);
+  });
+
+  it('rejects out-of-range offsets', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'GMT+15' });
+    expect(result.success).toBe(false);
+  });
+
+  it('rejects completely invalid timezone strings', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams, tz: 'Not/A/Timezone' });
+    expect(result.success).toBe(false);
+  });
+
+  it('accepts undefined tz (optional parameter)', () => {
+    const result = streakParamsSchema.safeParse({ ...baseParams });
+    expect(result.success).toBe(true);
+  });
+});
diff --git a/lib/validations.ts b/lib/validations.ts
index 749fb1ea1..487ee112f 100644
--- a/lib/validations.ts
+++ b/lib/validations.ts
@@ -127,12 +127,54 @@ function dimensionParam(name: string, min: number, max: number) {
     .transform(toDimensionValue);
 }
 
+/**
+ * Maps raw GMT/UTC offset strings (e.g. "GMT+5", "UTC-3") to the
+ * Etc/GMT±N format that Intl.DateTimeFormat accepts.
+ *
+ * Note: The Etc/GMT sign convention is *inverted* relative to the
+ * common GMT± notation — Etc/GMT+5 means UTC−5. This function
+ * performs that inversion automatically.
+ *
+ * Returns the original string unchanged if it doesn't match a raw
+ * offset pattern, so callers can pass any timezone string through.
+ */
+export function normalizeTimezone(tz: string): string {
+  // Match patterns: GMT+N, GMT-N, UTC+N, UTC-N (whole hours 0-14)
+  const match = tz.match(/^(?:GMT|UTC)([+-])(\d{1,2})$/i);
+  if (!match) return tz;
+
+  const sign = match[1];
+  const offset = parseInt(match[2], 10);
+
+  // Validate offset range: UTC-12 to UTC+14
+  if (offset > 14 || (sign === '-' && offset > 12)) return tz;
+
+  // GMT+0 / UTC+0 → UTC (avoids the Etc/GMT-0 / Etc/GMT+0 ambiguity)
+  if (offset === 0) return 'UTC';
+
+  // Invert sign for Etc/GMT convention: GMT+5 → Etc/GMT-5
+  const invertedSign = sign === '+' ? '-' : '+';
+  return `Etc/GMT${invertedSign}${offset}`;
+}
+
 function isValidTimeZone(tz?: string): boolean {
   if (!tz) return true;
 
+  // First try the timezone as-is (covers IANA names and Etc/GMT±N)
   try {
     Intl.DateTimeFormat(undefined, { timeZone: tz });
     return true;
+  } catch {
+    // Fall through to normalization
+  }
+
+  // Try normalizing raw GMT/UTC offsets to Etc/GMT format
+  const normalized = normalizeTimezone(tz);
+  if (normalized === tz) return false; // No normalization happened, it's invalid
+
+  try {
+    Intl.DateTimeFormat(undefined, { timeZone: normalized });
+    return true;
   } catch {
     return false;
   }