fix timestamp truncation (#3874)

soh/soh/OTRGlobals.cpp

@@ -1144,8 +1144,7 @@ extern "C" uint64_t GetUnixTimestamp() {
     auto time = std::chrono::system_clock::now();
     auto since_epoch = time.time_since_epoch();
     auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(since_epoch);
-    long now = millis.count();
-    return now;
+    return (uint64_t)millis.count();
 }
 
 // C->C++ Bridge