Use RTT midpoint to adjust Date.now

Measure local time before and after fetching server time and compute offset using the midpoint (compensates for network latency). Round the calculated offset and lower the adjustment threshold from 5000ms to 500ms to avoid applying small corrections. Also rename timing variables for clarity.

packages/napcat-core/helper/server-time.ts

@@ -14,15 +14,17 @@ export const OriginalDateNow: () => number = Date.now.bind(Date);
  * @returns 偏移量（毫秒）
  */
 export function hookGlobalDateNow (getServerTimeFn: () => string): number {
-  const localNow = OriginalDateNow();
+  const start = OriginalDateNow();
   const serverTimeStr = getServerTimeFn();
+  const end = OriginalDateNow();
   const serverTimeMs = parseInt(serverTimeStr, 10);
 
   if (isNaN(serverTimeMs) || serverTimeMs <= 0) {
     return 0;
   }
 
-  const offsetMs = serverTimeMs - localNow;
+  // 补偿网络延迟：假定往返时间对称，服务器时间对应本地时间的中点 (start + end) / 2
+  const offsetMs = Math.round(serverTimeMs - (start + end) / 2);
 
   if (Math.abs(offsetMs) > 5000) {
     Date.now = () => OriginalDateNow() + offsetMs;