Calculate and graph from varying input dates

I would like to graph the weekly electricity, water and heating consumption using Excel.
Normally this isnīt a big deal to do but I would like to be able to input the meter readings at varying times.
Thus I would need to include a function to calculate the divergence between the entered date and time and a fixed date and time (reference).
The attached file shows my Excel grid.
In other words:
Each week I input the meter readings for electricity, water and heating.
The day and time I input the data vary plus/minus 1 day and the Excel needs to calculate as if the data would be input always at the same day and time of the week - for example, Friday morning 10AM.
On the next sheet of my Excel file I would like to graph the weekly consumption for electricity, water and heating.
What would be the best approach, either by function or VBA code?

rgds, devplan