set file mtime to content based etag value with nginx-etag
npm install batch-update-nginx-etag``sh
npm install -g batch-update-nginx-etag
find ./ | nginx-etag --filter '.js$'
``
this provides a command that sets the modified time of all matching files to the value of the first 32 bits of it's md5 sum.
nginx builds the etag from the file's mtime and the number of bytes. if you serve the same static files from a distributed set of nginx servers the mtime has to match or your users will download content they already have.