diff --git a/packages/oauth-provider/src/token.ts b/packages/oauth-provider/src/token.ts
index 7d3d493edc..dbafe68347 100644
--- a/packages/oauth-provider/src/token.ts
+++ b/packages/oauth-provider/src/token.ts
@@ -21,7 +21,9 @@ import {
 	getJwtPlugin,
 	getStoredToken,
 	isPKCERequired,
+	normalizeTimestampValue,
 	parseClientMetadata,
+	resolveSessionAuthTime,
 	resolveSubjectIdentifier,
 	storeToken,
 	validateClientCredentials,
@@ -759,8 +761,8 @@ async function handleAuthorizationCodeGrant(
 
 	const authTime =
 		verificationValue.authTime != null
-			? new Date(verificationValue.authTime)
-			: new Date(session.createdAt);
+			? normalizeTimestampValue(verificationValue.authTime)
+			: resolveSessionAuthTime(session);
 
 	return createUserTokens(
 		ctx,
@@ -1063,7 +1065,9 @@ async function handleRefreshTokenGrant(
 	}
 
 	const authTime =
-		refreshToken.authTime != null ? new Date(refreshToken.authTime) : undefined;
+		refreshToken.authTime != null
+			? normalizeTimestampValue(refreshToken.authTime)
+			: undefined;
 
 	// Generate new tokens
 	return createUserTokens(
diff --git a/packages/oauth-provider/src/utils/index.ts b/packages/oauth-provider/src/utils/index.ts
index 67a7e82048..b460e989a5 100644
--- a/packages/oauth-provider/src/utils/index.ts
+++ b/packages/oauth-provider/src/utils/index.ts
@@ -60,6 +60,80 @@ export const getJwtPlugin = (ctx: AuthContext) => {
 	return plugin;
 };
 
+/**
+ * Normalizes timestamp-like values returned by adapters.
+ *
+ * Accepts Date instances, epoch milliseconds as numbers, and strings that are
+ * either ISO dates or numeric millisecond values such as "1774295570569.0".
+ */
+export function normalizeTimestampValue(value: unknown): Date | undefined {
+	if (value == null) {
+		return undefined;
+	}
+
+	if (value instanceof Date) {
+		return Number.isFinite(value.getTime()) ? value : undefined;
+	}
+
+	if (typeof value === "number") {
+		if (!Number.isFinite(value)) {
+			return undefined;
+		}
+
+		const parsed = new Date(value);
+		return Number.isFinite(parsed.getTime()) ? parsed : undefined;
+	}
+
+	if (typeof value === "string") {
+		const trimmed = value.trim();
+		if (!trimmed.length) {
+			return undefined;
+		}
+
+		const numeric = Number(trimmed);
+		if (Number.isFinite(numeric)) {
+			const parsed = new Date(numeric);
+			return Number.isFinite(parsed.getTime()) ? parsed : undefined;
+		}
+
+		const parsed = new Date(trimmed);
+		return Number.isFinite(parsed.getTime()) ? parsed : undefined;
+	}
+
+	return undefined;
+}
+
+/**
+ * Resolves a session auth time from common adapter return shapes.
+ */
+export function resolveSessionAuthTime(value: unknown): Date | undefined {
+	if (value instanceof Date) {
+		return normalizeTimestampValue(value);
+	}
+
+	if (!value || typeof value !== "object") {
+		return normalizeTimestampValue(value);
+	}
+
+	const direct =
+		normalizeTimestampValue((value as Record<string, unknown>).createdAt) ??
+		normalizeTimestampValue((value as Record<string, unknown>).created_at);
+
+	if (direct) {
+		return direct;
+	}
+
+	const nested = (value as Record<string, unknown>).session;
+	if (!nested || typeof nested !== "object") {
+		return undefined;
+	}
+
+	return (
+		normalizeTimestampValue((nested as Record<string, unknown>).createdAt) ??
+		normalizeTimestampValue((nested as Record<string, unknown>).created_at)
+	);
+}
+
 const cachedTrustedClients = new TTLCache<string, SchemaClient<Scope[]>>();
 
 export async function verifyOAuthQueryParams(
diff --git a/packages/oauth-provider/src/utils/timestamps.test.ts b/packages/oauth-provider/src/utils/timestamps.test.ts
new file mode 100644
index 0000000000..f92f58c819
--- /dev/null
+++ b/packages/oauth-provider/src/utils/timestamps.test.ts
@@ -0,0 +1,59 @@
+import { describe, expect, it } from "vitest";
+
+import { normalizeTimestampValue, resolveSessionAuthTime } from "./index";
+
+describe("normalizeTimestampValue", () => {
+	it("parses epoch-millis text values", () => {
+		const result = normalizeTimestampValue("1774295570569.0");
+
+		expect(result).toBeInstanceOf(Date);
+		expect(result?.getTime()).toBe(1774295570569);
+	});
+
+	it("returns undefined for invalid values", () => {
+		expect(normalizeTimestampValue("not-a-date")).toBeUndefined();
+		expect(normalizeTimestampValue(Number.NaN)).toBeUndefined();
+		expect(normalizeTimestampValue(9e15)).toBeUndefined();
+		expect(normalizeTimestampValue("9e15")).toBeUndefined();
+	});
+});
+
+describe("resolveSessionAuthTime", () => {
+	it("reads createdAt from direct session objects", () => {
+		const result = resolveSessionAuthTime({
+			createdAt: "1774295570569.0",
+		});
+
+		expect(result?.getTime()).toBe(1774295570569);
+	});
+
+	it("reads created_at from nested session payloads", () => {
+		const result = resolveSessionAuthTime({
+			session: {
+				created_at: 1774295570569,
+			},
+		});
+
+		expect(result?.getTime()).toBe(1774295570569);
+	});
+
+	it("normalizes direct Date values", () => {
+		const date = new Date(1774295570569);
+
+		expect(resolveSessionAuthTime(date)?.getTime()).toBe(1774295570569);
+	});
+
+	it("does not fall back to updatedAt timestamps", () => {
+		const directResult = resolveSessionAuthTime({
+			updatedAt: 1774295570569,
+		});
+		const nestedResult = resolveSessionAuthTime({
+			session: {
+				updated_at: "1774295570569.0",
+			},
+		});
+
+		expect(directResult).toBeUndefined();
+		expect(nestedResult).toBeUndefined();
+	});
+});