Syntax error around converting timestamps to time

Question

I am writing a function that requires some comparison between the difference date and time parts of the timestamp data type. However it keeps running into syntax errors and I cannot find out what is going on. I looked into this post and keep having alternating errors like

psql:ERROR: operator does not exist: time without time zone >= timestamp with time zone and

plpgsql: No function matches the given name and argument types. You might need to add explicit type casts

Can anyone enlighten me what is wrong and what could be a direction I can head towards?

create or replace function riders_available_at_date_time(specified timestamp)
returns table(riderId integer) as
    $$
    begin
        select F.riderId
        from FullTimeRiders F
        where specified::date between F.startDate and F.startDate+27
        and exists (
            select 1
            from MWSWorkDays M join ShiftsInformation S on (S.shiftNumber = M.shift)
            where M.riderId = F.riderId
            and (specified::time without time zone between to_timestamp(to_char(firstStartHour, '99'), 'HH24')
                and to_timestamp(to_char(firstEndHour, '99'), 'HH24')) or
                (specified::time without time zone between to_timestamp(to_char(secondStartHour, '99'), 'HH24') and
                    to_timestamp(to_char(secondEndHour, '99'), 'HH24'))
                  )


        union
        select riderId
        from PartTimeWorkingHourIntervals P
        where specified between calculate_start_date(P.dayIndex, P.startDate)+P.startHour
            and calculate_start_date(P.dayIndex, P.startDate) + P.endHour;
    end;
    $$
language plpgsql;


create or replace function calculate_start_date(index int, starting date) returns date as
    $$
    declare
        position integer;
    begin
        select extract(dow from starting::date) into position;
        index = (index - position)%7;
        return starting + index;
    end;
    $$
language plpgsql;