fix: should return dates for expiration fields (#7351)

Co-authored-by: cubic-dev-ai[bot] <191113872+cubic-dev-ai[bot]@users.noreply.github.com>
Co-authored-by: Alex Yang <himself65@outlook.com>

packages/better-auth/src/api/routes/account.ts

@@ -484,17 +484,10 @@ export const getAccessToken = createAuthEndpoint(
 										accessToken: {
 											type: "string",
 										},
-										refreshToken: {
-											type: "string",
-										},
 										accessTokenExpiresAt: {
 											type: "string",
 											format: "date-time",
 										},
-										refreshTokenExpiresAt: {
-											type: "string",
-											format: "date-time",
-										},
 									},
 								},
 							},
@@ -590,14 +583,28 @@ export const getAccessToken = createAuthEndpoint(
 					});
 				}
 			}
+
+			const accessTokenExpiresAt = (() => {
+				if (newTokens?.accessTokenExpiresAt) {
+					if (typeof newTokens.accessTokenExpiresAt === "string") {
+						return new Date(newTokens.accessTokenExpiresAt);
+					}
+					return newTokens.accessTokenExpiresAt;
+				}
+				if (account.accessTokenExpiresAt) {
+					if (typeof account.accessTokenExpiresAt === "string") {
+						return new Date(account.accessTokenExpiresAt);
+					}
+					return account.accessTokenExpiresAt;
+				}
+				return undefined;
+			})();
+
 			const tokens = {
 				accessToken:
 					newTokens?.accessToken ??
 					(await decryptOAuthToken(account.accessToken ?? "", ctx.context)),
-				accessTokenExpiresAt:
-					newTokens?.accessTokenExpiresAt ??
-					account.accessTokenExpiresAt ??
-					undefined,
+				accessTokenExpiresAt,
 				scopes: account.scope?.split(",") ?? [],
 				idToken: newTokens?.idToken ?? account.idToken ?? undefined,
 			};

packages/better-auth/src/plugins/open-api/__snapshots__/open-api.test.ts.snap

@@ -1260,13 +1260,6 @@ exports[`open-api > should generate OpenAPI schema > openAPISchema 1`] = `
                     "idToken": {
                       "type": "string",
                     },
-                    "refreshToken": {
-                      "type": "string",
-                    },
-                    "refreshTokenExpiresAt": {
-                      "format": "date-time",
-                      "type": "string",
-                    },
                     "tokenType": {
                       "type": "string",
                     },