you can use a multi list system, in one list put th echek every monday (four weeks its similar to one month), when you complete the four mondays or the four weeks, make a check list and, clear the npc list, its a confuse, and long road to follow, but its a good way, if you want to try this don't use many time, test wiht housr, if in four houers the npc make a clear list, this gone to work wiht day check.
Sorry for my english.