Upload the SRT file that you would like to synchronize
You may specify negative or positive values depending if you want to add or substract time to the original SRT file
This web app is open source. You can see its source code on GitHub and make it better.
You can download a native desktop version of this app for Windows or Mac.
The source code of the native application can be found here.
There is also a desktop Java version available to download from this link.