Using the sample timetable Star Strider created:
DT = (datetime([2017 01 01]):caldays(1):datetime([2020 01 01])).';
HP = ones(size(DT));
TT = timetable(DT,HP);
Get the list of months for each row in the timetable:
Determine which months are between May (5th month) and September (9th month) inclusive.
isInRangeFor0 = ismember(M, 5:9);
Create the new variable. Since isInRangeFor0 is true when we want the new variable to have value 0 and vice versa, we can just negate that variable.
TT.newvar = ~isInRangeFor0;