a diversion that occupies one's time and thoughts (usually pleasantly); "sailing is her favorite pastime"; "his main pastime is gambling"; "he counts reading among his interests"; "they criticized the boy for his limited pursuits"
Since a pastime is something you do to pass the time, you would expect a double [s] here. Well, there is only one. The second [s] was slipped through the cracks in English orthography long ago.