I am using the sheets iPhone app.

I have a cell with the value a formula =now()

if I leave that sheet open in the app, use other apps on my phone, and come back to the sheets app with this sheet still open, the time in that cell is the old time I was last in there. It doesn’t get the current time in this case. 

What triggers an update to that time? (I know closing and re-opening the file will do it. I know I can do it manually in editing - function- recalculate. Does the app ever do it automatically?

similarly what triggers a refresh from the server if I have updated this sheet from another device. Assuming in the meantime I left it open in the app on my phone while the other edit was going on.