"Timeline" is pretty good. It's one of those movies you see once and it's good, but don't buy it.
SPOILER ALERT
One of the present-day characters (Merrick, I think) falls in love with a French noblewoman and stays in the past--his sarcophagus is discovered in the present-day, along with a message he left for his friends and a description of the life he led.
SPOILER ALERT
I figure Merrick could help the French out with various technical things; after all, the old professor is forced by the English to make Greek fire and another stranded time-traveler became a noble and is the one who does the forcing (plus he helps in some other matters).