Ok. I have been timing my game all day today to see when the store actually restocked. it was to the minute 8 hours after I opened my game. Which means that I either turned my game at the precise moment that the store updated this morning. or the 8 hour clock doesnt start until you turn your game on for the first time during the day.
This seems accurate to me. I know I have only been getting 2 restocks a day. One when I first open the game in the morning and once in the evening. I have been watching very closely the last few days. Last night my store restocked at 7 pm. (Which means it should have stocked at 3 am and 11 am today). I was restocked when I first turned the game on (around 9 am). Nothing at 11 am. Then I was finally restocked around 5 pm (approximatly 8 hours from when I turned it on this a.m.)
Edit to add that I haven't paused my game or time travelled.