Tamagotchi 4u npcs

Hello

I recently began to play the 4u. Although I found many guides, there seems to be no info about the npcs (non playable characters) that appears both in the backyard and in the park.

Does anyone know at what time does the npc that request us to help him built stuff (using school skills and thus earning up to 2000 points) spawn in the backyard?

Same goes for the npc seller that is supposed to spawn in the park and to whom you can also sells your items to...What time? Are there fixed days or conditions just as the alien eco-triplets?

Thanks

I just started to replay my 4U a couple days ago, and I think there are a couple of times per day you will be able to help fix items. I've had both tamas help twice today. I'm thinking one of the times is around 5:00pm-5:15pm. It was 5:11 pm today when I noticed both had someone in the backyard.