If you turn off the snap to grid feature, the position (args.start etc.) will point to an exact point in the grid and it won't be adjusted to point to a cell edge.
So you will need to change the logic. Your original code would only work for snapToGrid = true:
args.start = args.start.addHours(-11);
For snapToGrid = false (and scale = "Day") it could look like this:
const endOfDay = args.start.getDatePart().addDays(1);
args.start = endOfDay.addHours(-11);