Filling in tentative times in a cell.

I'm trying to make it easier for my mother to fill in a spreadsheet i made and have it stored on my OneDrive folder. This way I can check and make sure she's taking her meds as needed just by glancing.

sometimes she doesn't do well with times. One med she takes when she gets up, then another just before bedtime. But she also has some that are based strictly on 8 spreads
What I did was create a name set with times that she can pick from a dropdown when she takes them. But what I want to do is be able to highlight the next cell with the time she is taking her meds, +8 hours, just greyed out. Like a form with a default value that disappears when they select the cell to enter the real time

I have trouble with this 1. because it's a time add, and I haven't quite figured it out, and 2. I'm not that familiar with Excel.

Is this a simple task?