Showing Time Watchers: The Greatest of These by Julie Reilly

Robert, a keen young time-travelling historian from the year 2361 AD, is out of his depth. His mission is to spend six weeks learning about a community of Mesolithic humans living near ancient Kents Cavern. Dropped into a world of violence, rape, ugly death and harsh retribution, six weeks suddenly seems like a very long time.

Yana, devastated after the death of her beloved father, has a bleak future before her. A mating to a man she despises, a baby every year, ending only with inevitable death after a short and brutal life. But when charismatic stranger Robert comes to her village, her life changes irrevocably.

Expressly forbidden from intimate contact by Chrono-HQ directives, Robert must try to suppress his unwelcome and unexpected feelings for Yana. When the six weeks draw to a close, heart-broken Yana can't live without him. She goes after him, only to see him step through the time portal and vanish. Uncomprehending, she follows, only to find herself in the twenty-first century, bewildered, lost and utterly alone.

Now Robert and his contemporaries must find her and send her back before irreversible damage is done to the timeline.