Fix invalid build timestamp on certain timezones (#15245)

Currently the build .zip has its time set to the Unix epoch, which
apparently causes problems on certain operating systems when in a
timezone that is behind GMT.

The build timestamp has been changed to MetaMask's birthday. Time
zone adjustments will no longer result in invalid dates.

development/build/etc.js

@@ -49,7 +49,10 @@ function createZipTask(platform, buildType, version) {
       gulp.src(`dist/${platform}/**`),
       // sort files and set `mtime` to epoch to ensure zip build is deterministic
       sort(),
-      gulpZip(`${path}.zip`, { modifiedTime: new Date(0) }),
+      // Modified time set to an arbitrary static date to ensure build the is reproducible.
+      // The date chosen is MetaMask's birthday. Originally we chose the Unix epoch, but this
+      // resulted in invalid dates on certain timezones/operating systems.
+      gulpZip(`${path}.zip`, { modifiedTime: new Date('2016-07-14T00:00:00') }),
       gulp.dest('builds'),
     );
   };