donations_2016 = gifts[gifts["date"].dt.year==2016]
donors_include = set(donations_2016["id"])
print(donors_include)

{1002,3043,4934, ...}

donations_2017 = gifts[(gifts["date"].dt.year==2017) 
                       & (gifts["date"].dt.month<5)]
donors_exclude = set(donations_2017["id"])
print(donors_exclude)

{2451,3047,4474, ...}

population = donors_include.difference(donors_exclude)