In the interest of ease, I was thinking of assigning the start of the Fever series time line in April 2010 -- this would be 5 years after the initial publication of Spell of the Highlander in 2005. I based the 2000-portion of the time line by the publication of Kiss of the Highlander and then following the events from that point since they overlap fairly closely. I confirmed on Karen Marie Moning's boards that the events in Fever take place 5-6 years after the events in the Immortal Highlander which takes place a few months after the events in Dark Highlander.
If you're so inclined, you can figure it out exactly by mapping the months.