Monolingual speaker of modern US English in stuck in a time machine without a translator. How far back would they be able to go in time, to both the present day US, and UK, and still be able to communicate in and understand the spoken language? Read the written language?
Other side of the coin, English speakers from throughout history are being transported to the 21st century, how far back would one have to go before the person would be unable to read the language? Unable to understand the spoken language?