Your webserver is Linux? In general, the restrictions are on the Windows platform, anything that works on Windows should work without problems on Linux.
Having said that, the first parameter of create_from_string() should be a valid date/datetime string, and "-1 month" is clearly not a valid date. The fact that it works on Windows is because of the workaround we had to implement to bypass the lack of support for strptime. So, bottomline, you're using it in a way that is not supported.
If you want relative time, you need to convert it to a timestamp first, and pass that: