Hi, I want to create a night Zentu but I don't know what time I should do so. Does anyone have any idea when I can do it?

When it is dark in shengui guo, I think it is 3pm to 1am

Does anyone know where/how to obtain a piece of glittering jewelry for the Zentu?

you could start your own thread we don't mind this in Subeta. There's a Restocking Shop that sells jewelry called All That Glitters.

Because this particular shop vends and is part of the quest cycle and folks can only wear each jewel once, you can often find jewelry from that shop cheaper than buying it directly from ATG by buying from other users. Try going to the fansite Subetalodge and look through their item database that shows what ATG carries and price a few things with the Shop Search.