Was it ever clarified what defined the five year period? Was the Enterprise sent out on a long voyage from Earth and five years is how long it would be before they returned to Earth? Would the current crew work together for five years and then be broken and given new assignments? Would the ship itself be decommissioned and replaced after five years? My question is basically what event was going to happen in the fifth year that was substantially different than what happened in the fourth year or the sixth year? (ST:TNG avoided this issue by saying they were on a “continuing mission” with no set time period.)
On a related note, the Enterprise was already in the midst of its mission when the series began (unlike ST: The Next Generation or ST: Enterprise). Was it ever established what year of its mission it was in?