INTCK( interval, date1, date2 ) 
Gives the difference of dates between date1 and date2

INTNX( interval, date, n <, 'alignment' > ) 

It increments the date by the n value

I am beginer,I have answered this at my knowledge.

the INTCK function counts the number of intervals  between 
the two dates 

where as INTCX function  advances the date or time values  
by a given interval and returns a date or time values

for example:

data xxx;
count= intck('week','1aug1998'd,'31aug1998'd);
run;
 proc print;run;
the result would be 5 (it represents the no of weeks)

data yyy;
count= intnx('month','1aug1998'd,1);
run;

proc print;run;
the result would be the 1sep1998. becaz it corresponds to 
the  beginning of the next interval.