change epochs from 2000 to around 1980

> 380n*365n*24n*3600n
11983680000n
On windows, we treat files before such epoch as non-exist

Author: bobzhang

src/disk_interface.cc

@@ -66,7 +66,7 @@ TimeStamp TimeStampFromFileTime(const FILETIME& filetime) {
   uint64_t mtime = ((uint64_t)filetime.dwHighDateTime << 32) |
     ((uint64_t)filetime.dwLowDateTime);
   // 1600 epoch -> 2000 epoch (subtract 400 years).
-  TimeStamp output =  (TimeStamp)mtime - 12622770400LL * (1000000000LL / 100);
+  TimeStamp output =  (TimeStamp)mtime - 11983680000LL * (1000000000LL / 100);
   if(output <= 0){
     return 0;
   } else {